A pregnancy test is the only sure way to tell. Often someone can see the possibility of pregnancy in their significant other by watching changes in their habits, appetite, etc. but it is not conclusive. If she is also worried about pregnancy, then her overall demeanor will probably be changed anyway just from the stress. Get a pregnancy test, and then of course if it's negative then a couple of forms of contraceptives that are always used would probably be a very good idea to prevent future scares.