@srp1975,
You should ask for a paternity test early and often if you unsure about whether you're the father. You should document your request as a written Motion or Request for DNA/genetic testing. These are usually (if not always) granted when requested early in the proceedings. If you have signed the Birth Certificate, or signed some other form of acknowledgement of paternity, then you may have a more difficult argument. If you have waited until the day of trial to request the test, if may also be too late (although I would think the Court would want to find out true paternity if there is a real question, so the child knows with certainty who his/her father is).