Get a protractor or a carpenter's level.
With a carpenter's level you will need to go out to a distance where the level, set on the ground, (or close enough to it), will point to the top of the pole with the 45 deg bubble indicating level. The pole height equals your distance from the base of it. OR
Take a protractor and walk any distance from the pole, get an angle that you are looking up to the top of the pole, (the same angle that Smart $ shows to the sun in the previous post), use the same formula-the Height = distance from the pole x the Tangent of the angle.
Just for kicks, do both, see how close you get.