Artificial Horizon testing

All

I have an RC Allen Attitude Indicator Model 26 (Artificial Horizon) fitted
in my glider for cloud flying.
It is powered by 14V from a stepped up 12V soaring battery. ie plenty of
power.
It has been levelled.

I am concerned about its current reliability when showing straight and level
flight.

Can anyone advise on any tests air or ground-based, with the instrument
installed / removed than I can carry out to determine whether this
instrument is functionning correctly or not?

Will I see anything if I take it apart?

Regards

Rory O'Conor