Experience, reviews online reputation, COA available on products.
even with COAs, half the time the COA is from an older batch and you have no idea whats changed since. Or the COA is from the Chinese manufacturer who they are bulk buying the whey from, so its meaningless.
Sure its probably something of an indicator, but really the only way to know for sure what protein you are getting is to stick with natural local milk, chicken, beef, etc.
For me for protein powder since its supplemental for quick digestion when I'm in a rush, I really don't care about quality, as i'm ALWAYS getting enough daily protein in from solid food source. So I either buy what is cheapest, or what I really like the taste of