You could put the tank and stand in the position you want and partial fill it with an one or two and then take some measurements from within the tank, if the tank has cover which goes over the edge of the tank like a jewel or has an edging strip I wouldn't bother unless there is a hell of a tilt.
the method is very old, one of the first leveling instrument. you should have transparent hose. fill it up by water (leve some space) and on both ends of the hose water level is the same. (you can stick the hose to the cabinet by tape, or one end only)
this thing and 3x4x5 triangle is one of the oldest instruments