No you can't. I ran into this problem and the vet told me the only way you can tell is genetic testing.
Can You Tell If A Dog Is Pure Bread By A Solid Or Non Solid Roof Of Their Mouth?
Generally a purebred dog will have a mottled mouth....meaning pink and black but this does not always hold true as a mutt can also have the mottled mouth. The best and possibly only way to tell is if you kept track of your pet's breeding during her heat cycle and made sure that the only dog she breed with was the pure breed that you picked for her. A vet may be able to tell you whether or not the pups have been breed true but not always!!!
Laughable about vets knowing the breed of a dog...it is a crap shoot if they are mixed as one never knows what genes will dominate the dog's features and characteristics. My mutt has a mottled roof and is a black lab mix, maybe Dobie, Beauceron, etc. My other lab is purebred and does not have a mottled mouth. My vet said there is no way of knowing by looking at a dog.
I'm not sure what you are talking about. The best way is to take the dog to the vet. Maybe you could find a website that would tell you characteristics of the breed that you think your dog is.