If you can still knead them into shape, I would imagine they're still good. If they crumble when you try to knead them, they're probably too dried out to use.
If they're dried out, you might try to revitalize them with a couple of spritzes of lube, then knead the cr4p out of them. That may bring them back to life. Just a thought. I've never had to try it.
I usually hit mine with a couple of squirts before I put them in a ziplock.
Rick