The MKI shooting was ordered by the battle commander in the AWACS who did not like what the Indians were doing.
Presuming the engagement is only using the fighters onboard RADAR.
The RADAR goes from search to fire control mode, with much narrower beam and different frequency. This change is apparent to the Warning receivers of the target aircraft, which begin to beep.
As the missile is about to be fired, there is a momentary dip in the power output of the RADAR (to avoid damaging electronics). That is also readily apparent to the other guys avionics.