OK Holmes...

You/someone has still gotta deploy this DLL somewhere yes ?

I take it your DLL will be placed on a server that has IIS (or similar) running as well as being an App/File server, therefore, you won't know if a terrestrial App is using your DLL by reference or the IIS Server/ASP is using it ?

if that's not what you mean then you're either gonna deploy terrestrially or Web in which case a regsetting in the deployment will suffice to identify in which environment it's being used.

Even in the first scenario above a environment variable would suffice.

FBP