You may have heard that supplements are not regulated by the FDA (Food and Drug Administration), that is true. However, very reputable companies do go to measures to ensure that their product is quality.
Supplements may or may not contain the ingredients that are listed below the nutrition facts. Regardless of the health claim that you are taking the supplement for.................. if it sounds to good to be true then it probably is, you at least want to be consuming a product that contains the ingredients you are looking for.
What to look for: products do go through registration with the NSF National Sanitation Foundation to ensure the ingredients are what they say they are.
As for taking supplements first figure out if what you are taking is doing what you are wanting and more importantly if you can consume it from whole food first. Consult a registered dietitian to help you determine the most safe and effective and my general rule: Keep It Safe and Simple (KISS)
Remember as with any healthy habit, you want to be able to adopt a healthy lifestyle. So if you cannot take it forever, the results will not last.