How can I tell when my dog is in heat?

Question: 

How can I tell when my tiny dog is in heat when there no obvious outward signs? I want to breed her but I can't tell when she's in season.

Answer: 

Often the behavioral estrus (heat) is characterized by the bitch's acceptance of mating.  She often stands with her feet firmly planted to allow the male to mount (also known as standing heat).  The tail is sometimes deviated to the side to allow for mating (also known as flagging).  If unsure, diagnostics such as vaginal cytology are possible (best done at your regular veterinarian's office or with a theriogenologist, a breeding specialist).  The changes in the exfoliated vaginal cells reflect the effects of estrogen on the vaginal skin and indicate at what point the bitch is in her cycle.