I've often walked down the street or through the halls and found myself looking at a girls butt and thinking to myself, "her butt looks good" or "eh, not so good" this is a common practice of guys. My question is, do girls look at guys' butts in a similar way, or what do girls look for?

I think your behavior is normal. My fiancé is older than you and still comment about a girl butt out loud, and to me.

I'm a girl and I'd take notice, may not verbalise it as much as men do but never fear... a good body never goes unnoticed ;)