Alanwsg,
The smoke detectors will set off various alarms dependent on the aircraft type but on all types I've flown on (737,757,767,777,A319,A320,A321) there are invariably audible alarms to those in the area, inc pax, plus various signals on crew control panels, flashing lights outside toilet and on call panels and an alarm in the flight deck.
Jsl
P.S. For people that are thinking of trying this, we know all the tricks and I think you'll find the courts take a very dim view when you are caught. As said before, people have died from this sort of idiotic behaviour and there is little we dread more than an onboard fire.