With the board powered off I was trying to check continuity between each pin on the end of each motor signal cable through to each motor plug but got nothing on any of them. To do it with the board powered on I'd really like to have an exact procedure to do it, test from x to y.You can use one output from the FC to check all the motors one by one. In this way, you can be sure about the workability of every ESC.
Thats the next plan, it was working the last time it was fitted, so I'll fit it again and try to verify that the new board is the issue.No way to do this, as you expected. There is no straight line to check continuity at all. The topic goes into deep chaos. My proposal is to put back the board with the defective ESC to see still all the rest is working.