Doesn't matter what the flight number is, with the exception of United Express always check in with the operating carrier of the flight. If it says operated by US Airways, you check in with US Airways. It is their plane, their boarding gates, their luggage handling, etc. Just a codeshare flight number with UA. You will also earn FF miles according to the agreement between US and UA. Your UA FF status doesn't mean anything to the operating carrier. If they are Star Alliance you will (or should) get the Star Alliance benefits.