Answer:
1) If she is a little moodier than usual.
2) If she is "wanting" your geldings or stallions (pushing against them)
3) If she is peeing a little bit but multiple times in a short period of time.
4) try breeding her with a stallion and if she kicks him, she isn't in heat, mares that aren't in season don't allow stallions to breed them!
Hope I answered your question!