how do you know when garlic's gone bad?

Ew.

jergins, what did you use the garlic for?

spaghetti with meat and butter and fresh tomatoes and garlic. i cooked the garlic and meat up together to make for a zesty garnishing! (i have no idea, i just threw it all in there, come what may. it was actually all way too greazy and gave me a headache. so, lesson learned. but at least it wasn't because of bad garlic)

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