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.