An after-dinner trick that could protect you on credit cards

Here's a new little after-dinner activity you and your partner can do the next time you're in a restaurant, if you have used a credit card to pay for your meal.

After you sign the charge slip, and after you remove the ''cardholder's copy, '' see if you can - without getting your fingers dirty - pull out the little carbon slips between the copies. (It helps if you first fold the carbons so the inky side is turned under.) Then you can either tear up the carbons or take them with you.

More people are doing this these days - and not just at restaurants - as the frequency of credit card fraud increases. A couple of weeks ago, for instance, federal law enforcement officials cracked a New York City credit card counterfeiting ring. People watching the evening news on television saw boxes and boxes full of thousands of credit card ''blanks'' waiting to be imprinted with legitimate card numbers and names.

Those numbers and names, police said, came from dishonest merchants, a few bank employees, and carbons retrieved from merchants' trash after legitimate purchases. Police said the cards were sold wholesale for $50 apiece, then resold for $250 to people who used them to buy merchandise.

Credit card holders are usually liable for no more than $50 of unauthorized charges if their cards are lost, stolen, or counterfeited, says Frances M. Simmons, an official of Visa U.S.A. Inc., who works with financial institutions to develop and operate card programs. They may not even be liable for that much if they report any loss or theft promptly. But if someone is using a counterfeit card with your number and name, it may be a month or two before you're aware of it. Even though you are liable for only $50, it's still a bother having to show it wasn't you who took that weekend trip to Aruba or bought the hang glider.

Visa and MasterCard are trying to do their part. MasterCard recently introduced a high-tech credit card with a hologram, ''micro printing,'' and other special techniques that either make the cards much more difficult to counterfeit or add more secret coding. Visa's card has these features, plus an additional invisible hologram visible only under a balck light, a special ''flying V'' that only authorized printers supposedly can make, and some code numbers that make sense only to the card company and the merchant calling in for verification.

MasterCard has already started sending out its new cards; Visa's will come in the next few years.

In the meantime, Miss Simmons says, cardholders should be doing more to protect their plastic. Removing and tearing up carbons is a good first step, even if you have to withstand scornful glares from some waiters and clerks who think you're indirectly accusing their establishment's employees of picking dirty carbons out of the trash.

You can also try not to let the card out of your sight, although this may be hard in restaurants. Sales personnel have been known to run off extra charge slips for their own use. Even if the forged signature is totally inept, it is often good enough to be costly, or at least inconvenient.

Be careful about giving your card number over the phone. A frequent ploy, Miss Simmons says, is for someone to call people and tell them their names have been selected for a prize and all that's need is their bank card numbers. Don't give it to him. Of course, if you order something over the phone from a legitimate, established firm, you have more protection; at least you know where to find the company again if there's a problem.

Miss Simmons also suggests that people check their wallets occasionally to see if all their credit card are still there. If one is missing, they should call the issuing bank or card company immediately. For this purpose, you should have a separate list of card issuers' phone numbers. For an annual fee of about only have to make one call to report a whole bunch of lost cards. Some divisions of the American Automobile Association offer this as a free service to their members.

If you would like a question considered for publication in this column, please send it to Moneywise, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. No personal replies can be given by mail or phone. References to investments are not an endorsement or recommendation by this newspaper.