Behavioral changes such as becoming clingy, overly affectionate, grumpy or more distant than normal are all signs a female dog is in heat. While this is broadly true of most female dogs, some show no behavioral changes while in heat.
Another way to tell if a female dog is in heat is by looking for physical symptoms, such as bloody discharge and a swelling of the vulva. A dog in heat may also tuck her tails as a way to protect the vulva. A dog may also experience an increase in appetite while in heat and may also urinate more frequently than normal.