Its a common situation. You can of course physically go to each machine in your company and check what is installed. But I'm a lazy DBA, or I try to be, and I want a solution that can let me have another coffee before lunch time.
If you do a Bingoogle search for something like "list sql server instances" you'll probably find these articles:
I did. And when I did all I could think was "Sweet! How easy is that!?"
Sadly not as easy as it looks.