I had to follow your link to get it: I hadn't realized that 57 is not prime. At least I'm in good company.

It looks like a prime, but can be caught with the second-simplest test: sum of the digits is 12, which is divisible by 3. Hence it's divisible by 3.

(The simplest test being of course if the number is even and bigger than 2)

Edit: now that I think about it, probably should not have tried to impose ordering to the simplicity of tests. There's of course the divisibility by 5 test, which is even simpler.

Take 111 as an example.