I buy Garden of Life's Raw Fit Protein powder from Vitacost at half the price of what our health food store sells it for. I trust the Garden of Life company. The product looks, tastes, and acts the same as what I used to buy in the store. It is in the same container with the same listed ingredients. Unless I have both samples tested by some 3rd party certified lab, I cannot say 100% that anything is authentic. However, I usually always buy products made by a company that I've purchased from in the past and that has a good reputation (like Garden of Life). Supplements are not FDA-approved, so I guess some corrupt companies could put sawdust in a capsule and call it a miracle weight loss product. So try to buy products only from companies you've used before and trust, that have helped you, that have some quality manufacturing label, are made in a country you trust (I buy USA-made products whenever I can, and read the reviews-skip reviews where people get something for free or highly discounted in return for "an unbiased (haha) review". Look up information online about the company and use your best judgement.

They sell familiar brand names as well as their own line of products and have a huge selection. You'll have to do your research on the products that interest you to ensure they are not coming from China.