How Can You Tell When A Dog Is In Heat?

Her vulva will swell, she will have a discharge which is bloody, and she will be attractive to male dogs.  She may also behave differently, that is more or less playful than usual.

Usually, the vagina will swell and you will either see a white substance or blood around the outer edges or actually dripping from the dog. Another good way to tell is if you are attracting a lot of male dogs while you walk her. :P

Yes, the rule of thumb has been ten days in ten days out which unfortunately means ,she could be in heat for awhile before showing any real signs such as bleeding . Mainly before she starts to bleed she might move her tail to the side or want to go outside a lot she also may lick her self a lot like she's cleaning but usually her area will be swollen , when she starts to bleed you might see little drops of blood on the floor or her bedding or lets hope not on your furniture, your local pet store will have little dog pads for her that you can use to keep blood off your stuff , however some dogs are not receptive to the pad thing so you might have to try something different like a diaper or old under ware with a hole cut out for her tail in the mean time if you don't want pups keep a close eye on her and by all means get her spayed  f.y.i. It cost more to have her spayed when she's in heat and it will cost you more if she's already with pups . Be good to her ,be a responsible pet owner if you don't want puppies.