I am trying to find a camper shell or topper for my truck. What is the right way to measure the bed?

You shouldn't have to do the measuring. The installer is probably better at that.
If you intend to buy a used shell, measure width for both the inner lip and the outer edge. Then measure from the inner edge of the front to the tail gate.
The shell or cap should fit onto the middle of the bed rail and the gate should latch securely when the cap is closed and locked. Buying used is risky and may require a test fit before buying.
You may also want to use sealing tape between shell/cap/topper when installing.
I hope this helps.