Although many metrics are available to report the inherent complexity of software, none is as intuitive and as easy-to-use as Tom McCabe's cyclomatic number measure of testing complexity. While it is not absolutely foolproof, it results in fairly consistent predictions of testing difficulty. Simply draw a graph of your program, in which nodes correspond to sequences of instructions and arcs correspond to non-sequential flow of control. McCabe's metric is simply e-n+2p where e is the number of arcs, n is the number of nodes, and p is the number of independent graphs you are examining. This is so simple that there is really no excuse not to use it.
Use McCabe on each module to help assess unit testing complexity. Also, use it at the integration testing level where each procedure is a node and each invocation path is an arc to help assess integration testing complexity.
Reference:
McCabe, T., "A Complexity Measure," IEEE Transactions on Software Engineering, Dec 1976.
