Go with your gut feeling, make sure, you can see mum with her puppies at around 6/7 weeks. Ask questions, about mum, dad age, how many litters, temperament.... anyone who knows their dogs, will be able to talk about them for hours!
This is the most spectacular naivety which is exactly how puppy farms operate
At 6/7 weeks, puppies will be quite independent from their mum. They won't be suckling and they will be exploring their surroundings. Mum will be happy to let them do heir own thing.
There is absolutely zero way of telling whether the bitch with them is their mum, or the placid stunt bitch who gets a revolving door of farmed puppies dumped with her to give the thin veneer of respectability
Ask all the questions you want. They answers will be what you want to hear, and you've got no way of verifying. Anyone can talk for hours about dogs when they are trying to make a few grand from the process.
The real danger is that idiots think that these sort of easily-faked steps somehow mean they aren't getting farmed pups.
Idiots will think they've ticked theee boxes, asked their basic questions and got a legit puppy. The reality is that they have poured more money into the pockets of puppy farmers.