I shoot an old Rollei 6x6cm and I too have difficulty in "seeing square". One aid that works for me is a small device I made from a piece of thin clear acrylic and two pieces of mat board that crops my groundglass to the "long" 8x12 format. When I'm feeling ambivalent about what format will reflect what I'm seeing I just drop the thing onto the finder and see whether it makes the image pop in a different way.

Or I just say "screw it", shoot square and crop in the darkroom!