Hm..i believe its actually just an educated guess.
Most of the times when dealing with diodes and mosfets, you have to guess and then work out the math to see if you are right.
Vds < vgs - vt for triode
vds > vgs - vt for saturation { both of these requice vgs > vt}
and vgs < vt for cutoff
Since the gate voltage is 5V and Rs goes to ground, you can 'guess' that Vgs > vt, so it it should be triode or saturation. If the math does not work out, then that leaves cutoff.