Place a straight-edge along the roof, extending over the top of the
gutter. If it touchs the gutter, and raises off the edge of the roof,
then the gutter is probably high enough. If the straight-edge lays
flat on the roof and passes over the top of the gutter, then water
rolling off the roof is likely to do the same - shoot right over the
gutter.

It is my understanding that in areas subject to heavy snowfall, that the
front edge of the gutter should be in-line with the roof plane. This allows
snow that builds up on the roof to slide off onto the ground, instead of
piling up on the gutter and ripping it off the building.

We get our fair share of snow in the winter, so I used this method when I
installed my own gutters. It seems to work well, even in heavy rains. The
only exceptions are the areas where the roof valleys channel the water into
a single area. In a heavy rainstorm, these concentrated areas to tend to
flow out past the gutter. I can't really raise the gutters up much, so I'm
thinking it has more to do with the gutter screens blocking the flow more
than the height of the gutter.

Anthony