Incline and dips - determining clearance

The clearance between my bed rails and truck bed is less than 6". This summer we stayed in a hay field in MT. The access to the field was less than good. On an angle, from the road dropped down quite a bit to the field. Driving in was no problem, the angle was odd, but the trailer followed just fine. On the way out, not the same at all the trailer came within about an inch of touching the truck. Thank god it didn't. I drove slow, but really wasn't prepared if it had hit.

I guess my advice is that there is no way to know until you do it. Just go slow and be prepared to stop and rethink the approach angle.