I have had this mammillaria for a couple years but can't tell if it's dead or alive. It seems anchored to the soil but hasn't really grown or changed in any way...any ideas on how to figure this out short of uprooting it?

If you've poked and prodded, and the plant seems firm, in addition to being well anchored, then I'd assume it's still alive. Sometimes you kind of have to assume everything is okay until you see definitive signs otherwise.