Quantitative Comparisons of Phylogenetic trees
I don't know whether it's exactly what you need, but there are formal algorithms for tree comparison.

There are basically two approaches: one utilizing tree lengths (branch score distance) and the other one dealing with topologies only (symmetric-difference metric): details and references can be found in the manual to treedist from the phylip-package, which implements both.

PAUP* has a command under the same name (treedist), which calculates the symmetric-difference metric only. This metric is very intuitive: it describes the total number of partitions (= splits) present only on one of the two trees.