I have a friend who just finished spinning a bunch of dog hair (well, about a small hank's worth, plied). I think her mom just collected it over a period of time, maybe kept it in a baggy. I don't think you really have to worry about it that much. But I could be wrong. I haven't had a lot of experience with prepping fibers.
I don't know if it's all that conclusive, but I usually do some finger spinning of a wool that I'm interested in - twist it between your fingers while pulling on it gently.
What kind of doggie do you have? My mom has a cocker spaniel mix and I would love to spin his fur, but she refuses to collect it for me. And unfortunately my dog doesn't have a undercoat that sheds much, so all I get from her are the coarse outer coat bristles.