how do you know when a dog is in heat?

Well, most likely you will see spots of blood when the female is in heat. Another way you can tell is by the sudden increased number of male dogs in your area. The symptoms of a female dog being in heat are a change in personality, a swollen vulva, and evidence of menstruation. This means your dog is ovulating.

Some dogs have "silent" heats. There is no outward sign of a heat cycle, the only clue you may have is that any male dogs have a suddung interest in her. Silent heats are more commonly seen in small dogs than large dogs.

Hope this helped! :)

~DogTear

When he starts humping stuff. if by heat you mean sexual urges.