I always go by the rule of if I don't know, I don't do. If it sounds dodgy it probably is. I have the advantage of a scientific background (this is why my ex-doc didn't like discussing anything with me), so I can work out what is in most supplements. Its just a matter of doing your own homework. If the stuff isn't properly labeled, theres a good chance whoever is producing and selling it, has something to hide. Beware!!!!
Times like these using the energy surrounding you and inside of you to intuit and discern is also really useful. Like any skill the more you practise the better you get, and the easier it comes to you. :)