Shit, its quite possible that she could be...

you can get the morning after pills (which works 72 hours after), but the later you take it then the less chance it has of working and its not even 100% sure to work anyway.

If shes not on the pill then theres a chance she could be...

Get her to take a pregnancy test (those are quite accurate but even those aren't 100%)

The only thing that is 100% is not having sex.