Methods and systems for detecting counterfeit cashless instruments are disclosed. A pattern of attempted redemptions or other transactional recordings of gaming machine printed tickets, vouchers or other cashless instruments having identification numbers with valid predictable fields but invalid unpredictable fields can indicate a likely counterfeiting attempt or operation. Occurrences can include a thief or other unscrupulous party discovering which gaming machine printed ticket fields lend themselves to prediction and which do not, and then attempting to guess at some of the randomly generated numbers in hopes of coming up with a valid number or number set. In addition, stored hash numbers can be compared to a hash number generated according to a one-way hash function. A pattern of invalid hash numbers can also indicate a likely counterfeit attempt.