How do I know if my girlfriends pregnant?

We just finished and we used a condom. But when i took it out there was white stuff on the outside of it. I checked the condom and it hadnt split at all, and the ejaculation was very runny (sorry for the content :S) but on the outside it was lumpy. How is it possible there is stuff on the outside? I checked carefully and there arnt any signs its mine, but i dont know?? Is it her? Should we be worried about pregnancy??