Honestly I think a lot comes down to understanding materials and some logic.

Natural materials tend to be desirable and can be combined and even bolstered with synthetics to make a nice blend. 98% cotton with 2% spandex for women's denim that has to stretch more? That's reasonable. But 12%? Adding in some random 15% polyester? That sounds like a combo that is trying to save money by cutting down on higher cost materials... Already a red flag.

As for logic, I have to ask myself, is the product I am holding trying to take advantage of a trend by "duping" an existing product? Like a thick fuzzy sock that gives the impression of a thick merino wool sock but is actually pure synthetics? I'm not trying to get down on synthetics here, they have their place, but it helps to ask-- what are they trying to lure me in with?

I know there is a lot we don't know... One company's bottom of the barrel cotton against beautiful long staple cotton are both still just going to be "100% cotton" on a label, but taking the time to look at a product in detail can help a lot.