[Misc] How do you account for confounding variables when trying new skincare?

Beauty Tips For Body Care
Say you're trying a new moisturizer/serum/toner, but you a few days later you see new breakouts or red patches. How do you figure out if it was caused by the new addition to your skincare regimen or some other external factor? Common ones: unhealthy eating, dehydration, seasonal changes, unknown allergies, stress, hormones, etc. etc.I guess you could wait out the product until "external factor" is gone but 1) Trying new products would take forever 2) if it ends up being the product's fault, could damage skin for a while. Maybe there's an easy solution I can't think of right now, but I'd love to hear everyone's thoughts!This is my first Reddit post so please excuse any post issues (:
Girls Blog 2015
Submitted by bberrios1