Holding wings level with the ailerons and trimming for no change of heading, or using the autopilot in heading mode and trimming for zero bank will accurately trim the rudder. You then need to disengage the autopilot (if used) to find out if the ailerons are unloaded or if they need trimmed. If after all this you have a beuatifully trimmed aircraft but the ailerons aren't neutral you either have a significant fuel imbalance or the airframe isn't straight - the latter is commoner than you might think!!
Using the autopilot in heading mode only works if you have a series yaw damper (permanently engaged), like most transport aircraft. If you have a parallel yaw damper, i.e one you have to disengage for t/o and landing, then all you are doing with the autopilot on is loading up the servo until eventually it will give up and disconnect. On the other hand, in theory this type should always fly the aircraft in balance so you only know if the aircraft is out of trim when hand flying it. Sadly the aeroplane often hasn't read the theory well enough and doesn't do this....
