In thinking on this topic, while preparing for a GAFE Workshop for Science Teachers, I thought I'd put together a Tutorial Video that explains how to make a Key using Google Forms. The great thing about Forms is that there is an option for Multiple Choice questions call "Go to Section Based on Answer". By using this feature, you can direct the user to the next question by using their response to the question. Perfect for a Dichotomous Key.
For more information watch this video tutorial.