68 gmc bent frame or suspension sag?

[Hudsonaj36]

Hi,

I have 2wd 1/2 GMC shortbed with rear leaf springs. In relationship with flat ground you can see that the rear of the drivers side is about a inch lower and this was confirmed by measuring from the ground to the frame. But where the bed meets the cab appears to be level with the rear window. So how can I take the suspension out of the equation and verify wether or not the frame is tweaked or the suspension is saggy and causing this.

Thanks