Often, out of sheer desperation I will end up enabling "Everyone" access on a folder that a web app is accessing (perhaps for file creation, reading, etc) because I can't figure which user account to enable access on.

Obviously, this is a very bad thing to do.

Is there a way to determine what account IIS is using at that exact moment to access folders (and perhaps other resources like SQL Server, etc)?

Are there logs I can look at that will tell me? Or perhaps some other way?

I usually use Windows Auth without impersonation. Not sure if that information is relevant.