I haven't found a switch or something like that, but the back of the circuit sometimes has a tiny "12" or "24" printed on the back. So it's maybe "factory specified"?
I haven't got a better picture, sorry, but you can see in white "24" above the gold colored esa mark. (you might need to zoom in a little :) )
It doesnt help that the original longines manual that came with the watch shows a '0' for the first digit. '08.00'. So the client thought the time should be in '24' hours. Does your 24 model state 8 o clock at '8.00' or '08.00'?
I think the only time, or rather occasion when you get "08:00:oo" on the display is in Chrono mode. I think all of the examples in that manual where the display starts with a "0" are taken from the Chrono mode, whether this is clearly stated or not.
Its a silly manual anyway - does the quartz crystal really beat at exactly 786,432 times per second?! Bizarrely 24x the frequency that the manufacturer ESA claim for the movement in their 942.711 Tech Pub.
--Very interesting note from dutchsiberia about 12/24 hr PCB markings