But just because it shows conductivity does not mean it's a sufficient return path. Remember Ohm's Law, V = I * R. A circuit that flows small current will tolerate a higher resistance, but a circuit carrying significant current requires low resistance to prevent voltage drop and heat. Grounding is not really the correct term, it's return meaning the path for current back to what forced the current to start with. The feed and ground conductors must both be sufficient to carry the current that is not used for work and not impact the circuit much.
Problem is a handheld DMM is not accurate enough to really measure, so you have to do some of this analytically and verify workmanship with a test tool. IOW, if you know your current and target voltage drop, you can calculate the size of wire you need. Say a winch cable, where you want resistance to be in the tens of milliohms. It's not practical to measure your return to verify, so once installed you measure and are satisfied that your meter reads zero. You know it's working, your connections are fine and that it's something less than the minimum resolution is of the meter. Thing is most all wires are going to look like zero ohms to a lot of portable meters, so unless they are exceptionally poor you can only say its go/no-go and have to engage the maths.
"Why does the U.S. care which flag will be hoisted on a small piece of land thousands of miles away?" -- Ron Paul