You really can't be sure that either dog is the parent of those pups without some kind of dna test.
I have a difficult time trusting anyone I haven't known forever, can you tell?
I guess, but I can tell when someone is feeding me a line of crap. The dogs that I buy (English Bulldogs) are very expensive, and the breeders cannot operate them as a puppy mill.
I spoke to the breeder of my dog before the pups were even born. I was there about a week later when they were still feeding off the mother. I picked out my puppy then with a right of refusal for another 4 weeks.
Maybe that's a bit overkill, but I wanted to know.