This is lame solution but might help: Export both workflows, open them in notepad and use file comparison tool or past them in Word and then use native word document compare. You can see the tags and actions that might be different.

There must be a better solution.

there is no such a tool.

you can, however, export workflow to a file and will get workflow definition in XML structure.

you then can compare these XML files. it will uncover you all the differences, but you will need to get familiar with that structure.

then, there is Workflow analyzer Workflow Analyzer Version 1.4.0.36 Released  tool that parses that workflow XML structure and provides several different views on it.

it neither compares workflows, but can provide good guidance on workflow XML structure internals

Both of the above solutions are about as good as you can get I think. However, just obviously keep in mind that you will see all the config differences in the environment in the workflow, so any web service calls etc. will point to different URL's. So you'll have to fight your way through that stuff to identify differences. I very much doubt there's a better way though at the moment.