Can I respectfully disagree with the above and advise you to find the joist? We should try to get these little jobs right, for our own satisfaction and for wanting to create a home where things are done properly.
I wouldn’t be happy fitting a smoke detector into plaster. At some point you’ll have to change the battery. You’ll probably have to give the detector a good hard twist. Do you really want to worry about bringing a load of ceiling down on your head?
I’ve never needed a detector thingy – I’ve taken my cues from the position of light fittings and knocking on the wall/ceiling and listening out for the dull knocks which tell you you’re knocking on wood.
As someone has already said, if you drill and miss, it’s not a disaster – just a bit of filler required. Only screw to the depth of the plasterboard. Then, you will either hit wood or fresh air. Don’t keep pushing if it’s obvious you’re pushing into fresh air. Ceiling cavities contain wiring and plumbing 😉