Dumb advice, but it's served me well when I've been thrifting. Just because something is a good deal, doesn't make it a good deal for you.

I've weirdly been more dissatisfied with clothes I've bought that were either on sale or thrifted where I've said "I'm not crazy about the look/feel/fit, but I can't pass this up." than I have been with some more expensive clothes that I really took my time/save up for. And I have some really good thrift/consignment buys. Not knocking that at all, just saying that you can pre-Kondo things sometimes.

Having said that, I try to find a 3 way or 4 way seam, and pull gently on the different pieces of fabric that lead into the seam. If you have a sense that the stitching is going to pull loose, put it back. Read the laundering directions and make sure that the thing isn't dry clean only when you didn't expect it to be.

I'd avoid thrift rain gear - even the good stuff has coatings that wear off and it's very difficult to tell.