Answer:
That's a little hard if you're plain looking at it (and you're comparing it with all other people in the world), but if you are a woman, comparing with a man, and it looks pretty darn...bloody (menstruation), then you know it's YOURS and not his. On the other hand, menstruation and what comes out is not really urine, but if you just mean waste of any kind coming out from your body, then that applies.