have a look here for similar question How to check whether the workflow is triggered on New Item or Edit Item?

Thanks but I'm dealing with a document library--he basically answers the question for a listitem.

I'm not aware of a difference between list and docu library in this regard.

if I take Collin's (correct) answer, you can just the same way create a flag/status column in docu library and apply the same solution.

I would say, my suggestion from that thread, of having the main workflow with all the business logic being stared from two other workflows, one triggered at new document library entry and the other on document entry update, is applicable too.

or do you see any specific problems with applying one of these approaches?