can Mega Evolve!
Yes, in Platinum there's a checker in the Veilstone Game Corner prize building (no such NPC in DP, though, IIRC), and in the Celadon Game Corner prize building in HGSS.
I can confirm that the Hidden Power person is not in Diamond or Pearl.
Just to add if you battle a Kekleon, it's ability will tell you what type your HP Type is, as it matches type from the last hit. It's Possible to work out the type by battling certain type of Pokémon and noting the effectiveness. (For example, HP-Fighting will be super effective against on Normal types and not very effective to Psychic types.
PsyPokés has a guide, intended for LGFR, that can help narrow down your type: http://www.psypokes.com/dex/hp.php