Try the water bottle next time you are up to see.
clearly any acceleration will impart a distortion from the g vector. however at constant velocity the water surface will remain parallel with the surface of the earth.
obviously the water bottle is not a substitute for the avionics. but it does make a quick and dirty sanity check for which way up you are.
do you have any better suggestions?
thats why 3 orthogonal gyros are used to overcome any acceleration from movement.
if you want a more advanced solution you could have multiple gps receivers at extremities of the plane to compare timing pulses on the gps signal to figure out attitude to a v high precision.