AC also have conflict alert, whereby the flight data callsigns 'flash' on the radar for the a/c concerned. It isn't mandatory to act due to the amount received, quite often those being very spurious. (ie. both a/c in level flight and have been for many miles but one maybe wavering within 200 ft of actual level.) Pilots will know if a controller has seen a 'potential' by being told to 'maintain FL*** on reaching, traffic...' or suddenly told to stop climb/descent and you then see another a/c pass 1000/2000ft away!!