Waypoint Missions

Hi , its my first time here so I’m not sure what are the posting ethics here , sorry if I posted in the wrong forum.
So my issue is , I followed the instructions here http://ardupilot.org/dev/docs/raspberry-pi-via-mavlink.html , and managed to connect raspberry pi with the pixhawk. I am at a stage now where I can already remotely send commands to the UAV from the ground while it is in mid air using Putty. But now I have an issue that I have no idea how to solve. My objective is to drop a payload once the Copter has arrived at the last waypoint , and a Python Code has been written for autonomous takeoff. But now , I need to know when the Copter has reached the final waypoint. Is there a parameter that is changed somewhere when the Copter has reached the last waypoint that I can extract using a python code? Thanks in advance !

ArduPilot will send the status text message saying ‘Waypoint X has been reached’. The other option is to have your code check that your vehicle is within a target radius of the waypoint you need to trigger on.

Thank you very much for the tip ! Would there be a page to assist me in doing that?
My end product should be something like
if waypoint6 has been reached -> Drop payload

Any solutions to this? I need the same mechanism for my boat. I need to know when a waypoint is reached so my RPi can start sensing data. Thanks in advance!