I was looking at some grocery store cactus the other day...they looked unusually healthy, which surprised me because they were living in a light-less, air conditioned environment. One had at least two snails on the spines. Are land snails usually a cactus pest?

Also, a different, less healthy looking cactus had lost many of its spines. Will these grow back?

One last thing, when you're buying cacti from anywhere other than a specialty nursery, how do you make sure it's alive? I look to see if there are roots visible around the drainage holes in the pot, but sometimes they can't be seen. It's kind of hard to pull most cacti out of their pots with bare hands, so any tips to make sure one isn't paying for a rotted shell? Thanks!