If your Yorkie female is not spayed, it is likely that she will go into heat at some point after she reaches 6 months of age. During this time, your Yorkie will exhibit certain behaviour traits to indicate she is in heat. She will also exhibit several telltale physical signs. Yorkies, as well as most other dogs, come into heat every six months. Learn your dog's cycle so it will be easier to diagnose when she is in heat.
Watch to see if your Yorkie starts to mount other female dogs. Your dog's sexual maturity will cause her to do this when she reaches her time of heat.
Watch to see if your Yorkie to see if she starts to lick herself. Dogs that come into heat will frequently lick their back ends to keep themselves clean when they are in heat.
Lift your Yorkie upside down and examine the dog's vulva. If the vuvla has tipped forwards, this is a good sign that the animal is in heat. You will likely notice a very strong distinct smell when you do this, which is the odour the dog excretes to attract males.
Look for any red discharge that indicates she is going through a heat cycle. Like the human menstrual cycle, dogs will excrete blood. This is one of the easiest ways to tell if the Yorkie is in heat.
Stand your female dog next to an intact male dog to confirm that she is in heat. A dog that is ready to mate will "flag" the male by moving her tail aside for ease of entry. This behaviour is an obvious sign that the dog is in heat.