How to know if a supplement/vitamin/medicine contains what it says?

Thats it, Im buying a lot of stuff to deal with problems with my gastric bypass and possible non-detected T3/T4 problems. Now Im not sure on how to know if what Im buying has what it says it have or if its just encapsulated wheat flour. Im browsing at online stores right now looking for DHEA and there are so many brands and the price differences are so big, that Im quite concerned about what to buy.

What can I look for in order to rest assured that product has what it says? Are there any certificating entity I can check on to see if there is a file about that specific product from that specific vendor, that Im about to buy?

Buy from third party verified supplement companies and reputable distrubitors.

Realistically, the more famous a company is, the more they get tested. Even Arnold's company got slammed before and had to fix course. However, with the bigger names come more opportunies for fake products from smaller distributors.

But yeah man, hop online and do some research into what products have the best track record.