My cat has a mass behind his shoulder. How can I tell if it's cancer or an abscess?

Take him to the vet

God, I don't want to sueeze it, he's always licking it. I don't want to hurt him.

Only a veterinarian can diagnose the difference. Definitely have your kitty examined by a veterinarian.

Take to vet either way..

If its squishy, its an abscess, if its hard its cancer.