I have a WF with conditional start, and I check a field to see if it has been modified from the previous value. That's built in to the action of the conditional start. So the idea is just to select which field(s) you want to check for change.

If the document was accidentally checked out then these users might just need to be aware of "discard checkout" option. That way the document does not commit the changing of the modified date. The checking in of a document would possibly trigger your workflow assuming it was configured to start based on document/item on modified event.

Somehow would need to implement logic into your workflow to ignore the event of "accidental check-in's". Which could be tricky differentiating accidental vs. valid check-in's. If this is frequently occurring, even though you've educated the users then might have to consider setting the workflow that is triggered on change to run manually instead of automatically.