Default How does T-Mobile define tethering?

I've seen some discussion suggesting that they simply check for a browser desktop user-agent. Some have suggested that, due to this, tethering with an android tablet doesn't count as tethering. Others have said this untrue or only partially true.

Has anyone tested with different user agents to see what is counted as tethering and what is counted as mobile data?

Even if this is true, what about email and other non-http data use? How does T-Mobile determine whether it is normal data or tethering data?