No matter if she is preggers or not, you need to exercise more caution in the future. The fact that you are even worried about it means that you've done something wrong. Find a Planned Parenthood in your area, get her tested (and both of you tests for STDs if you have had multiple partners), and then get her on the pill. If you use it correctly, you won't need to ever ask that question.
Relax for right now though. Depending on her age and history of her cycle, 1 day shouldn't be a big deal. If she is more than a week late, I would suggest heading for PP. If she isn't 18, then suck it up and talk to her parents and yours. If you are over 18, then, well, don't drop the soap.
Edit: Sorry about the cycle comment... I've hung out with WAY too many girls.
Ryan