It depends
Some immobilisers work by cutting the power to the starter motor so that it won't turn over (but you could still bump start it if you got it to move fast enough and chucked it into gear - this of course only works if it has a manual gear box).

Other immobilisers cut of the fuel supply. This is easier on modern engines where they have electronic fuel injectors etc. On older models they may have chosen to cut the power to the fuel pump (if it's mechanical, they cant use that approach).

So it is entirely up to the designer of the immobiliser as to what symptoms he decides to get the car to show when it's on. But normally there's a flashing light on the dash to give you a clue.
( heyrudi has just found this setting, Fri 13 Apr 2012, 13:22, Reply)