Because of the many ways to mark cards, no set of investigative guidelines can ensure the detection of such cards; however, the following guidelines can be useful. The suspected card deck should be squared to determine if all the cards are of the same size and shape; fingers should be run along the sides of the deck. The finish on the playing card backs should be checked to detect any difference in the type of surface. The cards should also be held obliquely up to a light or be observed under a magnifying glass to determine whether or not markings have been added over the material of the manufactured card. Key cards should be selected for careful study and comparison, being attentive to individual sections of the card. To determine whether or not an irregularity is due to normal wear, the same location should be checked on both the bottom and top of the card and on other cards of the same value. Another method of field testing is to square the deck in one's hand and flip through the cards. During this procedure, the investigator should observe only one small portion of the design at a time through one or more flips through the deck. This process is repeated until all small portions of the backs have been examined. This procedure is most effective if the cards are put in numerical order. 8 figures