Place it in a gym. Use GymHuntr.com to scan the gym. From a desktop or laptop computer view the gym and press the "Gym History" button. Click in the area under the pokemon's name with your name and the pokemon's CP (this part doesn't seem to work on mobile). It will display that pokemon's level, IVs, and moves.
Not really a viable process for scanning lots of pokemon, but works for the few that the typical IV calculator can't pin point.