how do you know when garlic's gone bad?

I find that the fresher the garlic, the harder it is to peel. Of course, now I have one of these, all garlic is easy to peel. But it's a good general rule.
http://img.epinions.com/images/opti/ae/2e/pr-Cooking_Tools-CHEF_s_Garlic_Peeler-resized200.jpg