I recently acquired a large pile of assorted bare PCBs (about 20lbs of them) and I needed a way to tell how thick the copper on them was, so that I would know what to expect when etching them.
After considering mechanical methods of measurement, I decided that it would be easier to just measure the resistance of the copper, and determine the thickness from there. First I tried just sticking and ohmmeter across a piece of PCB but it couldn't reliably read the resistance; a 4 wire meter was needed.
Since I didn't happen to have a 4 wire ohmmeter in the lab, nor enough hands to touch all 4 leads to the pcb, I decided to make up a little probe that had the 4 contacts that I could connect to an ammeter/voltmeter to emulate the functionality of a 4 wire meter. So I cut out a little piece of pcb material, and dremeled it into a few electrically isolated but mechanically connected sections. I then drilled a few holes in it and soldered in some wires.
I used 2 short pieces of #12 wire and 2 long coiled up pieces of #22 wire to create the contacts. The #22 wires are set to spring up just higher than the #12 wires such that the #12 wires can be securely pressed against the board, and the #22 ones will gently spring up against the copper. The idea is that a current source (about 1A works well) is fed through the #12 contacts into the copper plate, and a voltmeter is connected across the #22 contacts. The voltage read on a meter and is inversely proportional to the thickness of the copper (ie, 1/voltage = copper thickness).
This project was quick to make (about 10 minutes) and it worked extremely well. To calibrate it just press it against a piece of 1oz copper clad, and adjust the current to read a nice round value on my voltmeter meter (on my probe with a 1.5" contact spacing 1.5A gives 1mV-per-inverse-oz of copper). That's it! To use it press it against a piece of copper clad and take divide 1/voltage, the result is now the thickness of the copper.