Other than asking the man in Mistralton city Pokémon center and ingame
First, find your pokemon's IV using http://www.psypokes.com/dex/iv.php
Then, type it in http://www.psypokes.com/dex/hp.php to find out the type AND the power
It can be hard since you have to know how much EV you gave to the pokemon and the IV calc can tell you the range(for ex. 26-31)