How to tell if Windows Installer 3.1 was used or not?

From what I have seen is that if there's no .NET Framework on the PC then the Windows Installer 3.1 is required before the the .NET Framework get installed.

So, I'm working on creating a WISE Installer and I'm using the script. I thought for a moment, how do I tell if the Windows Installer 3.1 had been installed or not. Since Windows Installer 3.1 does all of the work so I have no way of telling what files and directory are installed. I only need to check to see if that directory or file had exist then I can tell the script to ignore the Windows Installer 3.1 and move ahead with the .NET Framework installation.

Thanks...