RSAF has aircraft on Alert 5 (or equivalent) status. Last year
there was a light aircraft on a round the world trip that
approached Singapore airspace with radios out of commission. Pair
of Skyhawks was scrambled to intercept for fears of the aircraft
being hijacked.
Secondly, as far as aircraft are concerned Singapore and Malaysia
airspace is so close that aircraft is "handed over" from one
civilian airport radar control to another, and origin of aircraft
is closely tracked. In order to air drop sufficient numbers of
paratroopers into the island I believe there would be a certain
list of parameters that radar operators are trained to look out for
(guessing, because I am not Air Force).
For example, formation of aircraft, not necessarily from the same
airport, but all converging onto a similar location (ie
Singapore).
I fervently hope that I won't have to live to see the day that
there is armed confrontation between Singapore and Malaysia, have
family on both sides of the Causeway, but I believe that
preparedness is what keeps the peace here. Kuwait got too
comfortable with Iraq, which was why the larger country was tempted
to invade.