If I root my phone, can my service providor tell if I am using my phone for USB tethering or as a WiFi hotspot?
Technically they can inspect the HTTP Headers and look at the User-Agent field to determine browser and OS; but imagine the millions and millions of HTTP requests for web pages and applications in a given day.

They can do trends, and data mining techniques to find out - but is it worth all that effort for them just to track down someone using a few more MB of bandwidth? Probably not.