Before you do anything else make sure you have a backup of all your files:
So if anything goes wrong there is always your trusted backup.
Over the history of iTunes there have been many attempts at doing this in many ways.
Winamp has this facility built in.....look below
just place a tick in "Automatically remove missing files" then click on "rescan now"
We can do the same with iTunes but via a java script written by apple.
I found this on the web and have tried it on my music library. It found about 200 dead links.
I have no idea how accurate this is but the script looks simple enough.
If you have iTunes open and have some visible deleted files marked with a "!" you will see them disappear once you launch the script.
The script is here:
http://ottodestruct.com/itunes/RemoveDeadTracks.txt
save it as a .js and then launch it.
Once it has finished you will get a count of dead files.