I don't have a power meter but if I did - I would find a steady steep hill about half a mile or so long, time myself going up and then do the math or use one of the online power calculators. Compare that to the power meter.
Long and steep so that you're going slow, minimizing the effect of air resistance and rolling resistance loss. It would more closely approximate mgh/t. (ie, mass times gravity times height/time) which is easy to measure.