Dogs usually come into heat about four times a year. They will have a bloody discharge and often search for a mate. Dogs should be at least two years old before breeding. If they are bred before the age of 2 it can have serious health consequences for both the mother and pups.
The most obvious sign is when your large breed dog starts to bleed during estrous. Additionally, he vulva will be swollen, and she'll be urinating quite frequently, sometimes two to three times as often as she normally does.