Fill power is figured out by dropping a measured weight into a tube of down filled with one ounce and then measuring the volume. There are pictures showing how this is done on the web. Search under fill power or down, something like that.
pics

ifdl

If you go to WM or FF's websites, they tell you how much down is in each of their bags. You could use that as a comparison, even if the down you have is lesser quality.