Where was it made?
Many dietary supplements are made with raw material from China or other countries with little to no safety regulations. Also, because dietary supplement companies have a more or less free reign in America, "made in the USA" does not guarantee high quality.
Is it marketed as a miracle cure?
Long story short, there are no miracle cures. From state-of-the-art medication to ginger tea for headaches, there is no one cure that treats everyone successfully. If a product is presented as such, stay away from it!
What about the quality of the ingredients?
Many companies choose to use low grade ingredients to boost the amount of ingredients while keeping the cost down. Just remember, more of a substance, does not equal better. Also, you often get what you pay for. Shopping on price alone will lead you to supplements that are made from animal waste or are contaminated through their manufacturing process.
Is the company that sells it transparent?
Often there are hidden costs or customers unwillingly get set up for automatic credit card payments.
Are you being offered a 'money-back guarantee'?
This often happens with low-quality, high margin products. high sales will offset the loss of returns. Also, companies know that not many people will go through the trouble of returning products.