The unfortunate answer to this question is that you might not be able to tell if a supplement is fake. In some instances, it may be obvious. For example, if you are deficient in a vitamin and after taking the vitamin supplement your lab values do not change, you should question the authenticity of the supplement. However, other times the answer might not be so clear. If you have taken a vitamin before with no side effects and suddenly you experience unwanted side effects, it might indicate something counterfeit. In addition, if the name and place of the manufacturer are missing from the vitamin bottle, it might indicate foul play.