CorSec Engineering has made a handy-dandy tool to help you check firing arcs. Check it out.
From them to you:
We have decided to release the firing arc portion of our Steampunk Mini Template as a separate item. The Firing Arc Tool is a small disk with etched arcs for 90,180, and 270 degree fields of fire. We feel this handy little tool will be useful in a wide variety of games. They are cut from blue acrylic.
Price: $1.99 each