How do I know if I've had an orgasm?

I know that sounds kind of crazy, but I'm honestly not sure. I'm a girl and I've had a few different experiences, never with another person; there have been a few dreams that might be wet dreams (they were never intense) and I've tried to get myself off without penetration a few times, but I can never really define that moment of "oh, that was an orgasm." Is it a full-body sort of feeling, or mostly in the crotch? Is there a definite peak or does it just sort of stop feeling as good? Does your head feel light and your muscles tense? I read up on it and there are so many different variations that I'm really not sure what to think. Based on what I've read, I don't think what I've experienced has been an actual orgasm, but just what leads up to one. My heart's never beating very quickly like I've always thought it would, for instance. So is there any sure way to tell if/when I actually have an orgasm? I'm not looking for techniques or anything, just info so I know what's going on.