I found an elegant way of measuring the focal length of any lens, including tele constructions:
In an otherwise unlit room I taped a sheet of white paper to the wall opposite of the window. I drew one vertical line on the paper. I then focused the image of the window on the paper, lined up one edge with the line, and drew a line down the other edge.
Having a few lenses of known length helps a lot, of course - any lens will do! 35mm, MF, LF, tele or retrofocus, close-up "filters" (+1 is 1m, +2 is 1/2m and so on); anything with known focal length.
Plot your known focal length vs. width of window image, connect with a line. You can then read off the approximate focal length of the unknown lenses.
Since walls and windows tend to stay put this is repeatable - until you move to a different house.
Half an hour's fumbling about with lenses and I had a neat list of 10 previously unknowns!