Arnold E:
Secondary instruments (which is the ONLY way you can tell a single AH has failed or determine which AH has failed if you have 2):
Turn cordinator & dg (compass in worst case) for lateral.
Power plus performance for pitch.
Big beef with my training was did a bit of limited panel work, but there was no training on how to diagnose a suspect insturment.
One AH failure I had was obvious - the thing was showing a roll rate of about 1400 degrees per second. You could even feel the vibration through the airframe.
werbil
PS - the most insidious instrument failure I've had was a partial blockage of a pitot tube - airspeed indicator was undereading by about 20 knots. At least with a blocked static system the constant altitude indication gives you a very good hint to treat the airspeed indication with extreme caution.