On a serious note, you need to get the dates right, meaning if she is pregnant is she the right amount of time since you two had sex (or at least look the right size, imean if she has a huge stomach something could be iffy)
How long after you had sex did she realise she was pregnant (like pickles touched upon if its before her period she may be lying or it might not be yours)
Pretty stupid but, but did you realise the condom broke or were you told it did (maybeshe removed it and you never checked, hey it happens) Usually condoms are pretty dependable if used and put on right.
As mentioned, again you need to ask for a DNA test (behind the mothers back if possible. In the USA i'm not sure if thats possible, so you dont cause any animosity (sP?) if said baby turns out to be yours.)