How to tell if you’re having an orgasm?

How can you tell if you actually had an orgasm? With your partner and/or alone? I always question it.

Update: Self pleasure or sex, I reach a high point that’s very intense to the point of feeling like I can’t take anymore. I’ve had moment of legs shaking and moments of reaching that point without my legs shaking. Just trying to confirm if this is for sure an orgasm.