The degree of evaporation is the most effective way of determining if your boil is vigorous enough.
There is something to be said for the swirling of hops and such in a rolling boil, but the intensity can be guaged by what evaporates. As I said in an earlier post, most boils will do everything necessary, and in fact the perfect boil would be a waste of energy because we wouldn't filter out the resulting break material anyway.