Employees go through the canisters and pick out the obviously moldy ones. They're trying to work fast, though, so they can miss some that aren't obvious. The decay can also get much, much worse just in the time the product sits on the shelves, depending on how good the refrigeration in that exact spot is.

Anyway, open them up and examine before purchase. The employees will not mind, it's what they'd do too.

edit: Assuming there's no sticker. If they're sealed with a sticker then I wouldn't break it.