A swept wing airplane in a sideslip will tend to require non-zero roll trim to keep from rolling. The need to insert roll trim would be an indication of sideslip. Procedure to minimize drag is to insert rudder trim as needed to zero out roll trim.
A difference between heading and track angles as displayed on the PFD would be an indication of sideslip in the absence of any crosswind.
The need to hold non-zero bank angle in order to fly a constant heading is also and indication of sideslip. Wings level plus sideslip yields a flat turn (i.e., heading will be changing).