Shopping for a used car? Don't just kick the tires!

''It's the best car in town,'' says the used-car salesman as he pats the right front fender and beams at the onlooker. ''You won't find a cleaner car anywhere -- and that's a fact.''

Actually, he may be right. The car you're looking at was taken care of, all right, but not by the previous owner. The fact is, the car spent a little time in the dealer's reconditioning shop.

By and large, most used-car dealers make many of their sales on the way their cars look.

Usually, before a car is put on exhibition, the dealer will steam-clean the engine, paint the tires and floor mats, and replace armrests or the windowsill on the driver's side and maybe the steering wheel and pedal pads. He'll also vacuum the interior and put some polish on the body.

The car may also get a tuneup, battery check, and brake and transmission adjustment -- all for an investment by the dealer of $60 to $75. The retail markup could be worth $200 or $300 to the dealer, at the least.

Sure, these trifles are important to anyone buying a used car, but they are just the frosting on a possibly stale cake. You need to know what the cake tastes like before you buy.

You should start any used-car search by examining your need. Why do you want a car and how do you intend to use it?

Before you ever set foot on a used-car lot or even look at a private owner's car, you ought to consult the current Blue Book Values of Makes and Models. If you have a trade-in, you can see the retail value of your own car. Figure about 20 to 30 percent less than the retail value if you trade. A bank or credit union will often let you look at a Blue Book.

Don't be in too great a hurry to buy when shopping for a used car. An impulsive purchase may leave you repenting in leisure later on. You may have a good mechanic friend to take along to give you advice and help you get a feel for the prices and conditions of the various cars you inspect.

Then, when you narrow down your selections to a car you might buy, check it out in detail.

While the general appearance is not always conclusive about a car's condition , you might get some ideas on how hard the car was used and the way it was cared for by the previous owner.

Notice the external doors and fenders and the general condition of the body.

To discover hidden body repair, step to the front or rear and sight down the sides and across the hood, top, and trunk. Look for ripples and waves. Untouched body panels are almost perfectly smooth. Daylight hours are best for this inspection. Artificial car-lot lights at night can be deceiving.

Watch for repainted body parts and unmatching colors and overspray on chrome parts, inside doorposts, hood, trunk, and wheel walls.

Most cars more than a year or two old have had slight fender bumps, but evidence of extensive work on overlapping parts is bad news. Check door alignments with the rest of the body when opening and closing doors.

All cars can be expected to develop rust eventually, although the manufacturers are trying to delay the process as far as possible. Older cars are especially liable to rust. Cars rust from the inside out. Small blisters on the body panels are the tipoff. Probing them with your finger, a key tip, or coin would show a more serious rust condition underneath.

Crucial spots are often on the bottoms of the fenders, around the headlights and taillights, on splash panels, below the trunk lid, just above the rear bumper on station wagons, sometimes along the top of the fender edge where it meets the hood, and especially the rocker panels under the doors.

You should also be skeptical of cars that are in too perfect a condition for their age; in other words, without some nicks and scratches. A new paint job should make you extra cautious.

The most serious body-frame defect is ''dog tracking.'' A car ''dog tracks'' when the back wheels do not line up with the front wheels or the body and the wheels are headed slightly to one side.

Squat down behind the car and sight the rear wheels with the front end. Better yet, have a friend drive behind you when you road-test the car.

Before you finally do road-test it, however, inspect the interior for wear and tear on the upholstery, flooring, rugs, and other features. Compare the odometer reading with the condition of the rest of the car. In certain states it is illegal to set back the odometer.

Now you're ready for a road test.

The test drive is the time to decide if you like the general handling and performance of the vehicle. This is the time to listen for drive-train clanks, clinks, whines, and strange noises and rattles. Driving over rough roads should provide you with an idea of the overall condition of the body and suspension, the shock absorbers, and the springs.

Check the power steering from lock to lock by turning the wheel all the way over. The feel should be very smooth, with no squealing except a little noise when held at either lock.

The best way to check engine rings is to allow the car to decelerate from 45 miles an hour to about 15. Then step on the gas. If the rings are worn, the car will smoke. That could mean a $100 to $150 ring job.

During the driving test, also check the radio, heater, lights, and other service devices. With the car thoroughly warmed up, you can check the engine.

Pull the oil dipstick. If the oil is whitish or has white bubbles, a cooling-system problem exists. With the engine running, take off the oil-filler cap to see if a little vapor emerges. If blue puffs come out, the car may have problems. Put your ear close to the filler. If you hear a loud ticking or a heavy slapping noise, the car may have piston problems.

With the engine idling and now warm, hold a matchbook cover over the exhaust pipe. The free end should flap steadily and not be sucked in. Pull the transmission dipstick (on automatics). If you get a burned-rag smell, the engine may have more problems.

A good idea is to take the car to a qualified mechanic as well. If you do business regularly with the same garage man, he may look over your prospect for little or no cost. A diagnostic testing center will do a thorough check for $20 to $25. Be sure to get the owner's or dealer's permission first.

Don't expect a used car to be showroom-perfect. Parts of a car wear out just as any mechanical piece of equipment does during an average life expectancy: muffler, 1 to 2 years; fuel pump, 2 to 3; and shocks, clutch, brakes, carburetor , generator or alternator, voltage regulator, starter, water pump, and battery, 3 to 4.

Be prepared to replace some of these parts in the car you buy. A need to replace them is not necessarily an indication of major trouble.

Also, in buying a used car a warranty can be a confusing agreement. The first question is, does the car have a warranty or guarantee? Buying from a private seller gives you no such advantage, even though you may get the car for a lower price. Nor will you receive a warranty on a used car bought from a dealer ''as is'' or ''with all faults.'' Buying a car that is labeled that way is a gamble.

A car of recent vintage should have a factory-issued warranty. Then you will want to transfer the warranty to your name for the rest of the warranty period. Usually the dealer makes a slight charge for this service.

If the dealer offers a warranty on a used car, read it carefully, including the fine print. Many used-car guarantees are on a 50-50 basis, with the dealer paying half the repair costs and the owner the other half. If you don't understand all the implications of the warranty, get some help from a lawyer.

Above all, don't take oral promises. ''What promise?'' the used-car dealer may reply later on.

When you talk about the price of the car, make sure you get everything down in writing. Again, don't take spoken promises. And do not sign a blank contract. Get the car's actual price in writing, including all finance charges and the interest rate, the number and amount of installments, penalty for paying off early, down payment, and the value of any trade-in.

Make sure you understand all the charges and financing procedures.

You may also want to make any financing arrangements yourself, through a bank or credit union. Compare the interest charges over the full loan period. Then pick the lending institution that will be advantageous to you.

Finally, you may want to get in touch with the consumer protection division of the attorney general's office in your state capital so as to get all the legal facts that are available before you sign on the dotted line.

Simply take your time in buying a used car!