Is it possible to determine if a girl has had sex before by checking her nipples?

Nope.

Oh. My. God.

If you don't know the answer to that question you are too young to have sex. For that matter, if you have to ASK that question, you are too young for sex.

Is it possible to determine if a girl has had sex before by checking her nipples?

There is actually no way to tell if a woman is a virgin or not. There isn't any magical sign or whatnot that will be there or not be there depending.

Why is virginity important to you?

Yes, if you don't know her and say hi, and she pulls her shirt up and shows her mipples, most likely she has had sex before.

No, there is no way to tell if someone has had sex before.

I can tell you put a lot of trust in a budding relationship.