You can test for sex but not potency. You'll have to rely on the Breeder's honesty.
Your best bet is to look at someone's grow online. They'll usually mention the strain, but you might have to ask for the breeder. The downside is the batch may be different. Still, you can get great strains just by reading people's journals. But it's still a crapshoot. I did that for wedding cake and the buds were marbles. Nice weed, but terrible bag appeal. And I fucking hate trimming tiny buds, so most of it went in the hash bin.
You've seen people mention A Keeper. That's why. Find one for you, and keep it going.