iTunes database integrity check?

In iTunes I have a few ! that have appeared in the first column indicating iTunes can't find the file. So far I have found three folders (albums) that are missing from my music library disc and I don't understand how or when they dissappeared. I haven't found any individual missing files yet, just missing whole folders. It appears iTunes doesn't update the ! indicator until it has some reason to actually go open the file. Is there a way to automate this? So far I've been looking at each song with Command-I to check the Where info under Summary, or selecting the first song of an album and using Command-R to view the songs in finder. This is going to take a long time with nearly 8000 songs in my library. I'm trying to get an handle on the extent of the problem. I am careful to only use iTunes to manage the library (I don't move files around with finder). My library is on an external Firewire drive. Ideally, I would like there to be an "iTunes database integrity check" command.