If you can find a stray thread, or pull/clip one from inside the folds somewhere, you can put a match to it. Polyester will melt and leave a little hard black nub. Silk will burn and smell like hair, I think. Wes is the king of fiber identification, though - he can confirm.
Men's consignments are usually so niche that the people who run them know their stuff and deals - especially flippable deals - are hard to come by. It's the consignments that are mostly women's but have a men's section where the steals can be had.Awesome. You can see the excellent craftsmanship - that waist is closer cut than anything you'd see today short of Edward Green and above. The contrast with the bulbous-toe last is striking.