I think I might have gotten my girlfriend pregnant? What is the earliest way of finding out?

I know you can judge by a missed period, but how exactly does it work? because i know that girls period's aren't like clockwork and aren't exactly routine? we had a week ago, but as far as i knew, i didn't , or pre-, and it wasn't for long, but i want to be sure as i'm really worried now, even though it was my fault in the first place