The first way to tell if a traveler's check is legit is by the paper. Generally, fake traveler's checks have a thicker paper than real ones. A traveler's check is usually thinner than a generic banking check. The paper on a fake check also has a raised texture. This means you can feel anything that is printed on the check. On real checks, the paper is smooth like a bank check.
Real traveler's checks have water markings on the paper that can be seen when held up to the light. Those cashing the check should also look to see if there is a hologram line across the check. This should be rather dull, not extremely shiny. It should also be flat on the paper, not raised.
To verify that the check is being cashed by the proper person, the customer must sign the check when purchasing at the bank. The establishment cashing the check must compare a second endorsed signature (on the back) with the signature on the front. The customer should sign the check in front of the establishment, but if it is pre-signed, the signature should be compared with a photo ID.
