Doing some plumbing and I’ve pulled a couple old (but not antique) braided lines.
How do I check if they’re actually stainless? They’d be great filters for the mash tun and kettle that I’ve been planning to build.
After you cut the line to remove the inner plastic tube, and if one of the little threads pokes your fingers and you say “Ouch!” - then it’s stainless 
Hold a good magnet to it. If magnet sticks don’t use has steel in it probably a 400 style stainless which can rust. You want a 300 series stainless.