In my opinion, the best way to find out if they're any good is by simply installing them to see what happens. Play a few hours on them and see if they fall apart or behave adversely. Place the "test" bands in areas which are easy to reach should they fall apart or fail in some messy way.

I've pulled plenty of dried-out crumbly rings from games so that tells me they must have some type of shelf-life. Just like the leftover rubber bands from the newspaper - they just get old and loose their spiz I presume.

I keep them in ziploc bags in the dark.

Usually at shows you can get monster bags of them for $20 or $50.

I've found 10 year old ones, still in plastic inside of machines (with receipt yet!) and they were not as soft as new ones, but did not show micro cracks when stretched.

Rings are so cheap that if I had any doubts, I'd just throw them out.

For home use, at least you can keep an eye on old ones. Just don't use them in any hard to service locations, lol.