Marlin movement done

Hi,

I want to implement a filament sensor system for my marlin based 3d Printer. I think to use an encoder to detect if the filament moves when i call E AXIS movement gcode.(G0-G1).
My problem is to know when the movement was done.
I tried to check using planner.blocks_queued()==false condition but works only when the queue is empty.
Any ideas?
Thanks

There must be a pointer in the planner queue, that changes it's value when a new line off code is read into the buffer. Maybe you can place your subroutine there?