3 options there:
1. Take the vendors word for it, and compare to trip reports of what is reported as 6-APB
2. Get it tested by an independent lab.
3. Don't believe that its 6-APB
Basically, the only real option, is the one that nobody is going to front the money or resources to do. Even if they did, whats to stop it differing from batch to batch?
We are all playing a game here, and taking chemicals that we cant be 100% sure are what they say they are. Heck, when you take MDMA you don't get every single gram of it you buy tested. Your playing the same game with the online vendors as you are with a dealer. Unless you test it every time, you don't know for sure.
I get the point you're making, but how relevant is it? I think everyone understands that they are taking something called "6-APB" and that it could be something completely different. Basically, the best we can do is risk Bio-assaying and comparing effects to trip reported "confirmed" 6-APB.