Determining eyepiece focal length

I've got this homemade eyepiece that someone else made with no indication of its focal length. Is there any way to figure this out accurately? It looks to be a pretty neat little eyepiece except for the fact that it has a 1.1" barrel...I have no clue why. I guess I'll just wrap some tape around it or something.

---Daniel

Put it in a scope of known focal ratio and measure the exit pupil size. Multiply this measurement by the focal ratio. Voila!
--
Mike