Condition code for NAS or client

I want to condition code in a codeunit for file handling dependant upon whether the code is called from the Nas (server side) versus called from the client directly. If the latter I need to use a UNC path so want to use dotnet file handling but I can't use that for NAS as it give callback error so I want to use file virtual table for NAS.
This is in 2016. Ideally I need something similar to isservicetier. Any ideas out there?