Question

how to find out gm of a single stage common source amplifier ?

Answer 1

For BJT's
g sub m = collector current divided by V sub T ~ 26 mV
For FET's
g sub m = - (2 I sub DSS/V sub P) (1 - V sub GS/V sub P)

Answer 2

For FET's also
g sub m = -2 I sub D/V sub P

Answer 3

For your info
V sub T = KT/q ~ 26 mV at 25 degrees C
K - Boltzmann constant
T - Absolute Junction Temperature in degrees K
q - Charge |e|