For typical long channel, the condition for the saturation (active) region of NMOS is VDS > ( VGS-VTh).
For short channel we can use vdssat for determining the active region of NMOS i.e. NMOS is in active region if VDS > vdssat.
Can anybody please tell how Spectre determines the region (particularly Active i.e. region-2) for NMOS ?
There's a section which covers this (for bsim4 at least) called "Device Regions" in the Spectre Circuit Simulator Components and Device Models Reference manual
I am using BSIM-IMG model. Does spectre evaluates the region OP parameter (OP-57) for this model in the same way as that of bsim4 Model ?
I think so.