I use a dip stick made of wood. Pour in a gallon of water in your pot, put in your dip stick, where the water level is, carve a mark on your stick. Repeat this process until you get well above your normal volumes. Then when you want to know what your volumes are during the boil, just use the dip stick. I usually turn the heat down a bit so the boil isn't nearly as violent making it difficult to tell where the wort really is on the stick.
Ontap: Hard Cider
Primary: IIPA, All-Tythos IPA
On Deck: Munich Dunkel, Classic Rauch, Breakfast Porter, American Stout