The stuff could be every bit as good as the finest of UG labs operations. If your source is prepared to shoot himself with it (take your vial in for him to draw from) then he, at least, believes that sterility is not an issue, assuming he's even thought about sterility.
As for potency, you can send a vial off for testing. Of course, given the lack of regulation of his production, there's no guarantee that other vials from him are dosed the same as one you might get tested.
All you're left with is a word-of-mouth recommendation from the guy who is selling it to you. It's also unlikely that he'll give you the names of other people you can ask. That's one of the reasons for the existance of aas chat boards on the Internet. Some UG labs rise to prominance and get a reputation for making good gear and members will vouch for their products.
I guess I've just waffled a lot to tell you nothing you didn't already know. When it comes down to it, you take a risk every time you decide not to go with pharma-grade gear and then you still run the risk of buying fakes. If you really want to feel confident, buy your own powder; make your own gear and send some off for testing.