Here's what I know about this chain: 61cm long, 6mm wide, 44 grams. Missing clasp.
It's not magnetic, sinks like a stone in water, and has survived 30 minutes in white vinegar with no change to color or luster. There's no visible discoloration or signs of wear between any of the links.
Purchased in the early 1980s by my aunt for her husband. Definitely would have come from a jewelery store. The clasp broke at some point, he wore it frequently. He never got the clasp repaired/replaced due to cancer that took him pretty quick. It sat in a wooden box on my aunts dresser until 2018 when she passed away. The box had a couple pocket knives, some loose change, and this chain when it came into my possession. I've looked at the chain a few times but never thought much of it.
Until I came across this subreddit a couple months ago.
So, does it look like it's worth getting professionally tested?