Read out tachometer from PC fan when manually turning the blades ( driven shaft )
Internally the tacho pin is wired as an open collector


(source http://www.bearblain.com/fan_speed_control.htm)

With no supply connected to the red wire the base of the transistor can't be turned on so it never pulls down the output (collector).
The only way I see is to open the fan and tweak the circuit to power only the tachometer section and not the motor itself.



You can always create an individual circuit to measure the RPMs.
One of the options is with a Reflective Optical Sensor like



Here is a sample project