I don't think there's anyway of telling really, you just need a vet to confirm it so you are doing the right thing by taking your hamster. If it puts your mind at ease, i found a large red lump on my rat, gizmo, just under one of his paws. I took him to the vet and they just cut it off and now he is fine. Is your hamster still eating and drinking and is it still his usual self? If he is then the lump possibly isn't much to worry about but your vet will be able to say for definate. Good luck and keep us posted.