TOPIC:

Is there a way (? using a multimeter ) to determine the number of turns in a 540 brushed style motor?

TBH I don't see how there can be, there are just too many variables - state of brushes/comm, strength of magnets (+ age/heat related diminution), number of parallel winds (single/double/triple/quad), efficiency & state of bearings (and what type they were in the first place), motor timing (degrees of advance/retardedation between brushes & magnets), and no doubt other factors I've missed - an that's just for no load testing, testing under load adds a whole 'nother layer of complexity.

Theoretically you could make a graph of every motor out there where turns/winds and current draw are published and compare yours - but what were the testing methods for all those motors, and to what extent were the figures massaged?

If you're trying to match an unknown motor to an ESC I think the best thing is to use a 12T rated ESC, as in practice the majority of old hopup motors (at least in terms of sheer physical numbers) tend to be in the 17 to 23 range.

Been tinkering with that too as I have several unknown cans with missing labels. A way to measure rpm would be handy and then compare that with the amps.

In the end, the number of turns doesn't matter that much, right?
What you'd want to know most is the max current draw and rpm...

You could test it on a dyno like Louisbaby here (or maybe not quite)...