You can't tell
If a girl tries to kiss your stomach, she may have a major crush on you. If you know her well enough to let her kiss your stomach, you probably know her well enough to ask her why she does it.
Just look under it's stomach, and if you see a little hangy thing then its a boy. And if it has nothing then it is a girl. You can also take it to a vet and check on the internet too.
... they look different?
You know when you have a baby girl your stomach is smaller than when you have a boy. Also its possible to know by the first month by going to the doctor, but the answer might not be accurate.
You look under you and if you have a you know what then your a boy.
She will look at you a lot.
eat less, exercise (jogging is wonderful), do crunchies
When you get nervious and and a tingling sensation in your stomach when you think of her. Good luck my friend
she will look constapated
Look down at yourself.
If a guy look at the girl twice then it means she likes the girl
You need to look in her eyes and... well... you just know if she is.