I wonder whether anyone has struggled with finding shorts in an unpopulated board?
Basically you need two things (could be three things depending upon what you have):
The first thing you need is a good quality DMM which can resolve down to 100 microvolts or better.
Secondly you need a DC voltage source (power supply) which can operate in constant current limit mode with very low output voltage. I have found that a current of about 1 to 5 Amps usually works best. Too much current may burn a narrow trace on the board but too little will not allow you to find measureable voltage drops. A high quality adjustable bench power supply can probably do this. If you do not have such power supply, you can use a fixed power supply which is capable of at least 1A and use something else to control the current. That something else can be an electronic load (if you have one) or just an appropriate power resistor. If your power supply produces 5V, then a 2 ohm resistor will limit the current to about 2.5A. Such resistor needs to be rated for at least 12.5 Watts which is (V^2)/R. The resistor or electronic load should be connected in series with the power supply output.
You connect the two shorted planes across the output of your power supply or (power supply + resistor or electronic load)
With a controlled current flowing through the short, you use DMM probes to measure the mV difference between the voltage on the two shorted planes. With one DMM probe connected to a fixed point on one shorted plane, you walk the probe on the other plane around to different points, trying to find a point that gives you the lowest mV(or uV) measurement. Then keep that probe there and walk the other one around, again trying to find the lowest measurement. That process should point you to the physical location of the short.