You really can't, if you're talking about a
fasttech clone, and are going with one of the more expensive ones, chances are that it will perform the same or nearly identical to the actual product. There is always the possibly of getting a tank that is dead on arrival, usually due to machining errors, but fasttech would replace it, so other than a time issue it would be well worth the investment. Chances are that you'll get a good RTA for a fraction of the price
EDIT: Since it's on topic, I should let you know, I am waiting for my kayfun lite from fasttech to arrive right now

I'm most likely going to stick to clones, just because I can't justify spending $100+ on a tank, I would for a good device like an itaste or the MVP though.