The simplest measure is to look at differences in quality and quantity of verbal expression when handwritten vs typed. Length of product (at it's simplest, word count), average length of sentences (proxy for complexity of sentence structure), complexity and elaboration of ideas. Words per minute. Those are a few of the elements I'd look for. There are a few other reasonably well-validated quantitative measures, but they're a little more laborious to record (e.g., correct/incorrect word sequences).
