I got a Rutland burn indicator from Ebay, also some cheap oven thermometers. Not extremely accurate, but very helpful. I know if my fire is getting too cool or ashbound when the stack temp goes below 150, and a little too hot when it goes over 200 on the indicator.
I bought a Rutland magnetic stove thermometer today to put on the side of the stove, so I can put my old Saftyscrew (sounds like something you would buy at a drugstore ) thermometer back on the stove pipe. Stove temp. is 300* and the pipe temp is 150*. That is doing prety good as far as heat loss isn't it