The JPL News article How NASA Will Know When InSight Touches Down is interesting and worth a read. It says that during the atmospheric entry, descent and landing, the Mars InSight probe will be transmitting simple radio tones to be received on Earth. The presence, strength, and mostly the Doppler shift of these transmissions can be used to monitor aspects of the trajectory and provide information on any issues if they arise.
Question: Why will it be Green Bank and Max Planck listening, and not NASA's Deep Space Network?
As a reference point I'll note that radio telescopes have been used to "listen in" on Mars atmospheric entries before: Was the time of Schiaparelli's landing chosen specifically so the Giant Meter Wave Radiotelescope could listen?
From the JPL News article How NASA Will Know When InSight Touches Down
As the InSight lander descends into Mars' atmosphere, it will broadcast simple radio signals called "tones" back to Earth. Engineers will be tuning in from two locations: the National Science Foundation's Green Bank Observatory in Green Bank, West Virginia and the Max Planck Institute for Radio Astronomy's facility at Effelsberg, Germany. Their results will be relayed to Mission Control at JPL.
These tones don't reveal much information, but radio engineers can interpret them to track key events during InSight's entry, descent and landing (EDL). For example, when InSight deploys its parachute, a shift in velocity changes the frequency of the signal. This is caused by what's called the Doppler effect, which is the same thing that occurs when you hear a siren change in pitch as an ambulance goes by. Looking for signals like these will allow the team to know how InSight's EDL is progressing.