:( how do I know if the baby is mine

Get a paternity test.
Is the baby born yet?
If not tell the hospital you want a paternity test before you agree to having your name on the birth certificate.

The only way to tell for sure is through a DNA test. There are certain precedents that establish legal paternity in the absence of a DNA test. Examples would be being married to the mother at the time of birth, signing an acknowledgement of paternity (usually done at the hospital, either immediately prior to or immediately after the birth) or an adoption order. Assuming that none of these applies in your case, then the only way you can assert your paternal rights and be held liable for child support is through a DNA test.

If you sign the birth certificate and aren't the father some states will make you pay support just because your name on the birth certificate is like admitting you responsible for the support. I don't know how easy it is to get your name removed if you prove not to be the father and your name is on it.