I'm not aware of a specific API. You probably will have to fetch the previous version's content, diff for yourself, and determine what's changed.
An alternative would be to add metadata during editing, e.g. while saving calculate some hints for the workflow system and rely on that data to go through the workflow. That would keep the messy part out of the workflow definition.
I'd recommend to utilize custom APIs in general for this purpose: Instead of messing with a lot of script code in the workflow definition, limit the script code to call your custom APIs - you'll have a lot better story there in debugging and deployment, IMHO. At least compare the options of script vs custom API.
