I have two Nikon SB800's and in college they have another two SB800's I can use. I bought both of my flashes brand new and are great but I want to get more to do more complex lighting setups. With Nikon's newest flash, the SB900, I am unable to buy a brand new SB800 so I want to buy some second hand ones.
The only thing I want to be able to test it, is there anyway I can test whether the flash has lost some of its power due to a lot of use. I would like to have all my flashguns holding a similar amount of power. I'm sure I can just test the flash against one of my own and see if there is much of a difference, but is there any other way that is technically better for comparing the power output of a flash?