How can you tell when a chihuahua is in heat my male keeps trying to mount her.?

A dogs heat cycle will last three weeks or so. The most obvious sign is that they bleed, however some dogs clean themselves so well, you will never notice this. Other then bleeding, you may see a change in behaviour, she may become more submissive to the male, she may allow him to mount when she doesn't usually. Unless you intend to breed, for the health of the dogs (not to mention the pet population) you should have them fixed and then you can avoid this problem alltogether.