Get a 45 degree set square.
Pace away from the building until, with one eye at one corner, the top and bottom of the building fit at the top and bottom of the set square (if that description makes sense).
Realise you've not been looking where you were going and disappear under a speeding bus.
Wake up in hospital end remember the number of paces you took, as that is equal to the height of the building (to within a few inches).
Vin