You'll want to search for a registry entry: set Registry Root to HKEY_LOCAL_MACHINE, Registry Key to Software\Microsoft\MSMQ\Setup, and Registry Value to InstalledComponents. Give the property a name and choose "Use In an Install condition", and supply the appropriate text you want to appear when the key is not found.
If you prefer to not stop the install right away, just store the value, and use MsiGetProperty somewhere early in your script to warn the user.
