With a buddy's help, just take it out on a sunny day and focus an image of the sun on a wall (Brick or stone for god sake - a 16" scope is a great fire starter) Have your buddy use a measuring tape to measure from the center of the mirror to the "spot" on the wall (watch the fingers!!! ow ow hot hot!!) I'd like to take credit for this trick, but its a John Dobson trick usually used for finding focal length during polishing on uncoated mirrors)