I feel the clothes and the seams. After a while you get a feel for premium materials and construction. I also look for known brands that make BIFL quality gear, but seriously the picking not up and feeling it test works great, if it’s an older garment and it is undamaged and feels sturdy, it probably is. Also check all the sweaters for anything 100% merino wool or cashmere, I have some pretty “nice” fancy sweaters I use as warming layers under my outdoor gear. $2 for someone grandpas cashmere sweater or $120 for a comparable smart wool.

If it has zippers check what kind they are and the brand of them, for mass produced stuff ykk makes the best zippers, there are a couple of zipper brands out there for high end and premium labels that are as good or better, but I’m not sure of all their names, but in general a ykk is a good sign.

For “waterproof” or outdoor gear, look to see if it is “seam-taped” by looking at the seams on the inside and checking if they have been sealed.

What kind of garment or gear are you looking for specifically, I might be able to give more specific suggestions.