There are two parts to my strategy.
 I can eliminate some lists, because I know there are factors for them: ending in 2 means it is divisible by 2 (not prime); ending in 5 means it is divisible by 5 (not prime). These come up because we work with a decimal system (all higher digits represent numbers divisible by 10, and therefore 2 and 5).
 I can look at a list of primes (e.g. https://primes.utm.edu/lists/small/small.html) and I see 1, 3, 7, 9.
This answers the question, with two sources.
Walter
