You will find it confusing and difficult to measure the speed from the 3 phase rectifier ripple even off load with the thing being driven at variable speed with wind.
On load you will not see the voltage ripple so you will need to look at current but again you will struggle. Bring down a pair of light leads from before the rectifier for your frequency measurement.
The ripple of a 3 phase bridge rectifier is at 6 times line frequency, so for a 50 hz supply you will have 300 hz ripple.
For 12 poles the line frequency is rotational speed divided by 10.
To measure the resistance of the alternator you will need the machine on the ground and you will need to measure between any pair of wires. You will need to do it by the volt drop method, multimeters are not capable of measuring these low resistances accurately.