NASA will know when InSight touches down on Mars when the spacecraft sends a signal back to Earth. This signal will be received by a network of antennas on Earth, and it will contain information about the spacecraft's status, including its location and velocity.

The InSight lander is scheduled to launch on May 5, 2018, and it is expected to reach Mars on November 26, 2018. The landing process will be extremely complicated, and there is a lot that could go wrong. However, if all goes according to plan, NASA will receive a signal from InSight shortly after it touches down on the Martian surface.

This signal will be a major milestone for the InSight mission, and it will mark the beginning of a new era in the exploration of Mars.