How to inspect on message send to ardupilot using mavlink?

Hello there,

I am trying to use optitrack to send position and altitude to a bebop2 running ardupilot. I think the message is sent successfully, but the drone did not react to it. It keeps showing ‘EKF2 still initializing’. So I wonder if I can spy on messages sent to ardupilot? to make sure which part goes wrong in my project.

Many thanks in advance!