There are several popping videos on YouTube. I certainly hope gently popping them cannot hurt them, I tried it with my baby, and while things "popped" I still can't be sure (if he is male he is not obviously male to the untrained eye). Another video/article I saw said if the animal appears female it is hard to tell unless you are an expert and can probe the animal, as it could still be male. But I am sure those with more experience can say whether that applies to adults as well as babies.

I thought this video by Ralph Davis was pretty good as he clearly explains what he is doing:

http://www.youtube.com/watch?v=BPmIpc_gP9I