You can check your Pokémon's happiness three different ways. By going to Verdanturf Town and visiting one of the houses, visiting a girl in Goldenrod City or by visiting Daisy in Pallet Town.
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you can check a pokemons happiness in Diamond/pearl by the use of a poketch app, in previous games you could only know in the happiness was maxed by obtain a ribbon from a certain character depending on the game.
Go to Vendanturf town Talk to the lady in the house beside "Wanda's House" Note: the Pokemon that you want to check it's happiness must be in front of the line.
get the Poke tech App Poke happyness
Golbat doesn't evolve in pokemon red, blue or yellow. Generation II, and onward, if Golbat's happiness is high enough it will evolve into Crobat.
He evolves at any level with MAX happiness(you can check your poketech app, the one with hearts over your pokemon. 2 hearts=max happiness.). But, you must level him up with max happiness during the DAYTIME.