The best way to work out L/D is to do glide tests.
Remember L/D changes with airspeed, you need to do several tests at different airspeeds to find maximum L/D, which is the one that allows your plane to glide the furthest distance without motor, from a given altitude.
You will need some data logging/telemetry equipment with GPS and airspeed sensors - if you are patient enough to log enough airspeed vs sink rate data, you can plot the polar curve of the airplane.
From there, you will know at each given airspeed, how much drag the plane is generating - hence how much thrust you need to overcome it.
For any given value of thrust - you can ESTIMATE power but it gets even trickier - as some people correctly suggested, thrust in the air and on a bench is not the same, since the actual angle of attack between propeller blade and airspeed decreases when airspeed increases - the prop "unloads".
My 2 cents