Ride as fast as possible up slopes that are 7-10 minutes long. Go easy for half that time and try again. Stop with a 10% time increase.

Your time will almost be inversely proportional to power, with 90% of your effort going into lifting the weight of you plus your bike up the mountain.

Speed variations due to wind won't dominate measurements as they would on flat ground where aerodynamic drag has a bigger impact on speed than your fitness, with speed increase approximately proportional to the cube of power as in a 5% gain increasing your speed 1.6% from 18 to 18.3 MPH.