How to tell if a frame is bent?

[Patrick Sullivan]

My bed leans to the passenger side at the rear, but it's flat(ter) in the front. It really started bothering me lately, so I figured I'd take the bed off and remount it. Looking underneath it.. all the bolts were tight, nothing loose or missing. The only reason I thought bed bolts was that it also felt very "loose".. the bed feels like it's hopping when you go over a bump.

So now I have no clue why it leans to one side. The bed itself is straight, as far as I can tell. The rear end of the trailing arms both sit the same distance from the ground. The coils measure within 1/4" of each other. The fact still remains that, measuring from the outside lip of the rear wheel well to the ground, the passenger side sits at 33 3/4", and the driver's side sits at 35".

So, I guess the only thing left is for the frame to be bent, huh? Is there anyway I can check for this? The frame isn't visibly tweaked/bent/wrinkled at any certain point, to my untrained eye anyway.