The Starlink satellite antennas all point at the Earth. The antennas are also small compared to radio telescopes. They also listen to specific frequencies. They would make the best radio telescope for detecting Starlink customers.

The customer antennas would make poor radio telescope because the antennas are small. If using array of small antennas as radio telescopes were possible, we would see them. Using interferometry requires measuring positions and times to sub-wavelength. The Square Kilometer Array is the closest, but it is small extent, with large dishes, and many low-frequency antennas.