how to measure kpd for a phase detector in a PLL?

Provided the phase detector has a finite phase gain (a digital detector may have infinite gain) you can supply two signals with a small frequency difference to the detector circuit in transient analysis (a constant slope dφ/dt ramp). The slope of output signal divided by dφ/dt gives kpd.

Which kind of phase detector are you using ? (mixer, exor, JK-FF, charge pump) ?
In any case, I would propose to use one single signal source for both channels and some kind of filtering (low pass or better: allpass) in order to create a controlled phase deviation in one signal path. Then, the PD output voltage divided by the phase difference gives the PD constant.

thanks for the help.
I am using Hogge's linear phase detector for CDR. so I will need a charge pump as well.