Probably the most subtle form of credit card crime today is something that credit card specialists call "misappropriation," which is the use of your card number without your permission.

Now, as the peak shopping season approaches, the risk of your number being misappro-priated is greater than usual. Here is how it can happen:

* You get a phone call from someone saying that they are from Visa, MasterCard or some other credit card company. The caller says he wants to verify your credit card number and asks you to tell him what it is. But in fact, credit card companies don't do that; it is a hoax designed to get your card number and charge goods and services to your account.

* You charge a restaurant lunch to your account and take only your copy of the receipt. The carbons end up in the trash can, where they are removed by someone looking for geniune credit card account names and numbers to make counterfeit cards or to order merchandise by mail or phone.

* You receive a phone call saying that you have won a vacation or a special investment opportunity but must give your card number for verification purposes. But the call is actually from a thief who wants your card number to make illegal purchases.

"Misappropriation has become as much or more a problem than outright credit card fraud or counterfeiting," said Meredith Fernstrom, a senior vice president of American Express.

To fight misappropriation and other forms of credit card fraud, which may be as high as $500 million a year, the credit card industry has been considering preventive measures, such as carbonless credit charges, Fernstrom said. Also, some mail order companies, after receiving telephone orders for merchandise to be shipped to an address other than the card holder, confirm the order by calling the card holder or sending a letter describing the transaction.

Credit card holders are protected by law against individual losses when their cards are misappropriated and they are charged for the unauthorized purchase of goods and services. Under federal law, once consumers report a loss or theft of a card, they are not liable for any unauthorized charges; in any case of loss, theft or misappropriation, their maximum liability is $50 a card.

But consumers may pay in other ways: The time that it takes to straighten out the problem, the inconvenience and the frustration. Also, credit card companies with fraud losses eventually pass those expenses along to all consumers in the form of higher prices for memberships and services.

As a result, industry and government now are working to alert consumers to ways they can help curb misappropriation and other forms of credit card fraud. One example of that effort is the leaflet, "Who's Got Your Number?" published by the Consumer Affairs Office of American Express in cooperation with the Federal Trade Commission. The leaflet explains the problem and offers these tips:

* Destroy carbon papers on card receipts.

* Save receipts to compare with billing statements and reconcile your card accounts each month, just as you would checking accounts, to make sure that the charges are yours. Report promptly any questionable charges to the card issuer.

* Never give a card number over the phone unless you are initiating a transaction with a reputable company.

For a free copy of the leaflet, write the American Express Co., American Express Plaza, Consumer Affairs Office, Department WP, New York, N.Y. 10004. A similar tip sheet can be obtained by writing the FTC Public Reference Branch, Washington D.C. 20580.