One method [Ward Cunningham] suggests is that of summarizing on a single line select punctuation characters from each source file. He calls this the file’s "signature". This will help the developer gauge the complexity of the code base.
He includes a few examples from files in WordPress - looking for the structure of curly braces and semicolons as delimiters for chunks of functionality. Included are a few lines of code to help you generate a signature from any file and an example of a result. It's an interesting idea, but I'm not sure how practical it is. It could be a helpful reference for how complicated the file is, though.