It was almost impossible to tell when my previous mare was in heat. She would get a bit goofy if she was near a stallion, but other than that, she pretty much was her normal self.
Dancer is pretty much the same. I'm sure she's come into heat since Rain was born back in June, but I sure haven't noticed much. Scooter (daughter's stallion) is in the pen right next to the one the mares are in, and Junior (two year old uncut colt) is in the pasture that backs onto both pens.
Its easier to tell when she's in heat by the way Scooter acts than the way she does. Scooter is pretty laid back, even around a mare in heat - he's just a little more interested in her than usual. However, whenever I've been watching, Dancer gives him the cold shoulder.
Daughter's mare, Mistie, is another story all together. She flirts, winks, pees, squeals and is generally a pain. And she's even spookier than she usually is, and that's bad!