[Product Question] How do you know that the products you're buying online from places like Amazon are the real deal and not just knock-offs?

Beauty Tips For Body Care
I didn't realize it but a friend told me that there's a big racket on passing off knockoff sunscreens, creams, and moisturizers, as high quality products typically purchased from dermatologists' offices. Because I've bought a few EltaMD products from Dermstore and Amazon, I'm unsure if what I'm buying is typically authentic if they're not licensed retailers. While the logical way to find out is to apply and see if I get a sunburn, it would probably make more sense to ask you guys here for your thoughts and experiences on fakes.
Girls Blog 2015
Submitted by Plucky_Cosmic_Relief