Verifying the overall proportion of your portrait drawing is reasonably straight-forward. You first compare the width to the height. Using the largest width (in this drawing from a to b), which is generally across the brow-line, compare it to the height of the face from the base of the chin (Mental Protuberance).

The width of my portrait drawing, when ascribed perpendicularly to the height, falls just above the hairline. The objective, then, is to ascertain exactly where that point is.

The rule of thumb is to always adjudge the shortest distance - this is most likely to be the accurate guess (but it does take practice and training).