One testicle feels "heavier" than the other. A hard (as in almost stone hard) bump that is part of the testicle (i.e. not part of the skin in the scrotum). Be careful, though, because the epididymis and the rete testis of the testicle can feel like bumps, but they are perfectly normal.
A tumour will not move independant of the testicle as the epididymis and the rete testis of the testicle will. This is sometimes difficult to determine before you have poked about so much that you have to stop though.