Dan' de Bourgogne, we can check the focal length of a lens, or a lens with an attachment such as a teleconverter, by setting an object on a table and shining a light on it. Then stand up a piece of white cardboard and lay a yardstick on the table alongside it pointing toward the object.

Hold the lens so that it looks at the subject. Start with it against the cardboard and move it slowly toward the subject. Watch the image that will be projected on the white cardboard. When the image becomes its sharpest, take notice of how far the lens has moved from the cardboard by looking at the yardstick (or tape rule) measured from the camera end of the lens. If the measurement is in inches, convert that to millimeters and that will be the focal length. You should be able to tell the change in focal length of the lens with and without the teleconverter.