I’ve posted before on this, but 4 ways:
1) if you are waiting at a core station - the train will enter at speed and only start braking halfway along the platform. Or, if you are on a train and it enters the core, and you are stopped short of a station waiting for the platform to clear, you will accelerate at full pelt into the platform towards a red signal.
2) On the train, if you sit in first class* behind the driver, you will hear the ETCS ‘beep beep’ as each movement authority comes through from Three Bridges, if not in ATO / ETCS then you will hear the bings and bongs of the AWS.
3) At core stations, a train in ATO will open the doors immediately after stopping. Under conventional driving there will be a (roughly) two second wait while the driver confirms she/he confirms the train is platformed and then releases the doors.
4) On the train, in the core, the acceleration and braking is ‘full on’ under ATO.
*First class ticket required on some trains, but on others the front first class is declassified. Usually those thay terminate at Sutton, Sevenoaks, Orpington and Rainham (Southbound), and St Albans, Welwyn and Luton (northbound). But some Luton services do have first class later in the evening, so watch out!