try this, you have to check the bundle identifier:
if not check_Application_exists("com.apple.iWork.Numbers") then open location "http://apple.com"
tell application "Finder" to application file id theApp
Uhm, This seems not to be easy. Let me undestand: how can I find every bundle identifier? My example talk about Numbers, but of course I need BI for every Mac applications (or maybe for the most, like Firefox, Photoshop, Illustrator, C4D, Office, Maya…)
And one more thing (I like to talk like Steve): this bundle identifier is not the file that I can find in myuser/Lybrary/Preferences, right?
(Yes, I know, I seem to be confused…)