you'll know, i cant explain it exactly, it's like all your muscles will tighten and let loose all at once.... you'll know..
I assume you're female, because for a male, there is a sure tell-tale sign: ejaculation.
For a woman, though, I can only tell you what I've read. Here is a clinical description of female orgasm:
<q><em>In the female, the third stage of sexual response is known as the orgasmic phase, which is both the most intense and short-lived stage. Studies have shown that during this phase, some of the vaginal and perineal muscle fibers undergo contraction.
If women achieve orgasm, it is most often the result of clitoral stimulation. In my experience, most women claim that the two most important organs for achieving orgasm are the brain and the clitoris. This does not mean that a woman has to be smart or well educated to have orgasms but rather that she has to be in the right state of mind. Most women also claim that reaching orgasm is a learned process and, unlike in the male, there may be various levels of orgasms.
Some individuals have suggested a role for the cervix in feeling a "deep" orgasm, however, most women report that bumping of the cervix during intercourse is an unpleasant feeling and can contribute to a cramping sensation after intimacy.
Because they do not become unresponsive to stimulation immediately after orgasm the way a man does, some women are able to continue having orgasms, one right after another. Many women report, however, that after the first orgasm, the clitoral area becomes more sensitive, and that any type of heavy touch can feel painful.</em></q>