You can always go around attacking other Pokemon with it until you find one that its super-effective against, then you can narrow it down. Like if I use it on a Zebstrika and its Super-Effective its Hidden Power Ground. If you have a Kelceon enter a double battle and attack your own Kelceon with Hidden Power. The Kelceon's ability will change its type to match the type of the Hidden Power.
It is calculated by a Pokemon's IVs. Serebii will explain in detail > http://www.serebii.net/games/hidden-power.shtml