The yaw is indicated by the slip/skid indicator. You adjust the rudder trim in flight to trim out any pilot rudder input. Also the slip/skid indicator can be used as an aid. The trim indicator really comes into play while on the ground so you can set the trim for take-off. In lieu of having a gauge I hold the trim for a while until it must be at full right for take-off and then adjust in flight as you make power changes.
It should have a gauge though.