Discover if Excel running in Browser

I have VSTO Excel addins (.net 3.5) and need to know when the Excel wookbook is being opened in a Browser rather than the Excel client. I haven't figured out a way to do this nor found a solution online and I suspect I am just not using the right seach words. Does anyone know how to do this?

Thank you

The process would either be Excel or a Browser. You could check each...

Thank you. I don't understand what you are trying to tell me. I can tell when Excel is embedded in Word (Word is the container) but I cannot tell if Excel is in the Browser (Browser is the container)EFT][/LEFT]