Question the customer about information on the ID - birth date, address, etc. Many don't bother to memorize the information or forget it under pressure.
Ask for a 2nd ID if you have any doubts about the first ID they show you.
Write the ID number in a notebook to verify later. This will also dissuade minors from trying to use a fake ID at your store in the future.
Follow your company policy or talk to your local police department about what to do with a fake ID. (Note: We Card does not recommend keeping the ID.)