The line in the sand for me was when I purchased new socks and, fresh from the sealed plastic wrapper, they just didn't have that glorious new-sock feel. Washings later I can still tell the difference between the counterfeits I got online and the real ones I got in a store, and it is always a little let down when they come up in the sock rotation.

I've bought the same Hanes underwear from different stores at different times and gotten very different sizes and material quality. Sometimes what seems like a counterfeit could be just manufacturer material/parts sourcing and factory variation.