To Test For... Do this... Look for this...
And Adjust This... Dihedral adjustment
A word of caution here: this test is devised for aerobatic planes. All trainers and many sport planes are deigned to have some rudder authority and extra dihedral for added stability. But for precision aerobatics, neutral stability is required and the rudder should not roll the plane. So don't try this on a Kadet Senior! :-)