Check for swelled vulva
During these times, it is important to check the vulva of your dog as it gets swelled. This is the best way of determining whether or not your dog is in heat. Remember that some dogs experience severe swelling while others only have marginal swelling. You should also look for other changes like the number of times your dog urinates during the day.
Check for strange behaviour
The behaviour of dogs also changes during this phase as most become unusually aggressive when they are on heat. This is due to the direct impact of the hormones in their system. Not to mention, some dogs often get irritated at unusual things which also changes their behaviour. When the female dog faces a male dog, she often exposes her vulva by moving her tail outwards. This way the dog expresses their availability. Note that some dogs often do this when the owner is rubbing their back as they get aroused.
Observe blood spots
One of the major signs of your dog being available for mating is by checking for blood spots. Here, you must go and visit the places your dog is most familiar with. You should closely look for the spots where the dog frequents. If you find any blood stain then it is a good indication that your dog is in heat. However, if the blood stains are barely visible, you must check the area around the vulva of your dog. This will give you a clue about the true condition of your dog. Remember, a blood discharge transforms to light pink from bright red during these times.