How can i locate all the songs in my itunes library at once?

How can i locate all the songs in my itunes library at once? I keep my library on an external hard drive (it is not always connected). Sometimes Itunes will get confused and give me the exclamation mark icon next to some songs because at the time it cannot locate them. This icon does not allow the affected songs to be synched onto my ipod (very annoying). I am able to click on songs individually to fix the problem but there are far too many songs to make this method efficient. PLEASE HELP is you know any shortcuts or have any ideas.