Have her take a pregnancy test. You can buy them at any pharmacy. Be with her when she takes it so you can see the "positive" or "negative" result!
Yes be very careful because sadly to say some females believe they can keep a man by lying about this which is extremely silly, but it happens all the time! I will tell you what my grandmother always told my brothers, Mommas baby daddy maybe!!! Basically the mother always knows its hers but the father should always take the proper steps to assure he is the father. Definitely make her take a test in front of you, make sure you see the pee hit the stick (make sure its a new one too.) Good luck!!
Nothing proves pregnancy like a pregnancy test. Drug stores have them over the counter, and perhaps there is a clinic near you that can do a back up test.
Why would she lie about it? What would motivate her to do that? Are you talking about breaking up or something? Strange in my opinion. Usually it is a relationship on the rocks or one in which it isn't even really a relationship in which a girl might fake a pregnancy or try to get pregnant.
Anyway, you can take a pregnancy test now on the due date of a period. Periods come on average every 24 to 28 days. These tests are very accurate. A false positive is almost unheard of (negatives more common) and they can capture the hormone produced from pregnancy VERY early. Get a test and have her take it. good luck