Side ribs (vertical and horizontal) are easy. Just wait for a calm cool morning and check out the condensation pattern on the exterior sides. Have some tape with you to mark them easily.
You may get away with that on the roof as well. Haven't tried that.
Your interior ceiling likely has 48" sheets (of something or another) installed lengthwise going side to side. Those joints should be over ribs. Ceiling vents and AC's should have ribs both to the front and rear of each.
It might also help to drop speakers, roof vent flanges, or anything else of consequence installed in the ceiling, and do some poking around above the interior ceiling panels with a piece of coat hanger wire, exploring front to rear. Any obstruction found is likely a rib.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake