This is a widely debated topic. Which is the timing of spay/neuter, and even if you should do it at all! Also there are options to the full de-sexing of a dog, that leaves intact the hormones. Such as vasectomies, and tubal ligation.
http://www.yorkietalk.com/forums/gen...if-ever-2.html
In terms of baby teeth, sometimes you will just see an absence of a tooth in the mouth, or sometimes you can see the adult teeth growing in.
5 months old is way too young to be spayed in my opinion unless there is an overweening medical reason for it ... such as an undescended testicle.