Consider the following AppleScript:
on is_running(appName)
tell application \"System Events\" to (name of processes) contains appName
end is_running
set
tell application "Finder"
set applicationsnames to get the name of every process whose visible is true
end tell
set appName to "Safari"
if applicationsnames does not contain appName then
say (appName & " is not running")
--add here what you want to happen
end if
return applicationsnames
This is returning {"Finder", "JavaAppLauncher", "firefox", "Microsoft Word", "iTunes", "AppleScript Editor"} for me
Hope this helps