I also found this video, by the very splendid Alan Thrall quite eye-opening. In it he, and then various members of his gym, undergo hydrostatic bodyfat testing. You get to guess the bodyfat percentages of the various people from a range of options. The results are quite surprising. I study this sort of stuff quite a lot but still managed to get most of them wrong, and the ones I was most confident on having rigth were generally the ones I was wrong on.