When is it done?

I am about to bake my first attempt at a whole grain pumpernickel, namely the gorgeous Any Grain Sourdough from PiP's blog.

His recipe has a very long baking time - 1h 40m - at fairly high temperatures. This is much longer than  I usually bake for - the highest percentage of rye I have tried so far is 40% .

How does one judge when these very solid loaves sre done? The bread is covered, so it can't be seen. Is there a target temperature or some other method to decide?

Thanks in anticipation, Patsy