one of the things I look for is the feedback from other buyers of similar products, assuming they'd spot a fake before me. So I'll read their feedback, paying attention to if that seller has sold a bunch of rare or niche stuff and if the other customers were happy too. In some cases I'm also more likely to buy from someone who doens't look like a churning businesses selling hundreds of random bottles (liquidator), but rather a person with a collection who's selling off a few and comments about how they've stored them or where they were purchased. To me - half used sounds promising, ie they just don't want it anymore. A few sprays, could mean they didn't like it, or it could mean something was wrong with it. New in the box is a mixed bag of potentials...
that's just one thing I do though.
have you used toolhaus.org? - you enter the seller's name and it'll show you all the negative feedback. It'll also show you feedback that is not longer listed or withdrawn...