Dear mohammad,

The threshold gate to source voltage of the MOSFET marks the on set of  drift conduction in the transistor. According to the idealized mos transistor model the drain current follows the well known relation in the linear region of operation:

ID= beta( VGS-Vth) VDS,

And in the saturation region:

ID= beta (VGS-Vth)^2/2,

THE symbols have their usual meanings

So either equations can be used to determine the threshold voltage, in the linear region one draws ID versus  VDS at certain VDS smaller than saturation drain vpltage VDSAT. The fitting with staight line must be accomplished at the highest current range such that the operates far from the suthreshold region. 

One can also plot ID versus SQ.ROOT of VGS in the saturation region and fits the a best straight line at the higher current range to operate the transistor far from subthreshold region.

In both cases the intercept of the  straight line with the VGS axis defines the threshold voltage.

There are other methods in the literature of the MOSFETS. However for sake of transistor circuit parameters the method above may be more proper.

For more information please see the paper:

https://www.researchgate.net/publication/220456459_A_review_of_recent_MOSFET_threshold_voltage_extraction_methods

wish you success

Article A review of recent MOSFET threshold voltage extraction methods

Other method is to select the gate voltage with the highest second derivative value of the drain current

Another one method is constant current approach, mentioned in ITRS guideline.