I've written a tool to tell us exactly that, called nginspex...hoping to open source it shortly. We use generated configs with tens of thousands of lines, for me this was an exercise in learning what nginx is up to as well as being able to say how a request will be handled.

The tool interprets the nginx conf, rather than compiling any of it (as nginx does with the rewrite rules), makes it easy to log which lines are involved in the processing as it hits them.