See if an application is running

tell application “Finder” to set itStatus to exists process “iTunes”

This will fail if the app name is different, which is common (on a mac it is usually a Bad Thing to rely on names or paths). I would rather use the creator code (“hook” in iTunes’ case):

tell application “Finder” to set itStatus to (“hook” is in creator type of every process) –> returns true if iTunes is running

If you don’t know the signature of your app, ask for it:

tell application “Finder” to return creator type of process “theApp” –> returns theApp’s creator code if it is currently running