For the most part the biggest indicator to me is the slight burning taste. It could also be when the juice feels harsh when you take a draw. Then, you dial it back to where it tastes and feels best for you.
As with any vendor, not everything they sell is going to work right. When I've had that experience with Brian, he moves on it right away. I've never had a bad experience with him, and I try to buy as much as I can from him because of it. Everyone I've recommended Brian's store to has come back with raving reviews about his customer service. He's also a forum member, although he's not on here much because he's focusing his time on his customers.