How to test the voltage of a speed sensor and identify the +ve, ground and signal wires using a DMM?
Normally most of speed sensor have 3 wire 
 1. Positive wire 
 2. GND wire 
 3. Signal wire

Now about your question you can't determine exact voltage level of signal wire using DMM because it's interrupt signal &amp; it's voltage level rise only for some mill seconds.

but you can determine which wire is +ve, -ve &amp; signal using voltage deference between each wire (from main board side). 

First of all unplugged connector and from mainboard (left in your picture) determine:


Voltage diff between light blue &amp; green wire(if it's 12V this means one wire is +ve)  
Then voltage diff between light blue &amp; dark blue wire(if it's 12V this means dark blue is +ve)  
Now using jumper wires connect each pin of mainboard to the sensor board (except middle one as most of time it's signal wire). if you rotate the sensor module (eg. bike tire) &amp; see the some voltage(as it's interrupt signal it's voltage rise only for milli sec) then this means middle one is signal wire.


You can also determine the speed as well using Arduino interrupt and voltage divider (if the signal voltage level higher then 5 volt)  

Note: I recommend you use oscilloscope etc to determine the exact voltage of signal.