There is a Poketch app called the Happiness Checker. Looks like this:
This function allows you to select Pokemon in your party and determine their happiness. You do this by holding the stylus down on them. 2 Full Hearts indicate maximum Happiness. When the Pokémon just has two hearts, it should be at an adequate level for happiness evolution.
You get it from a woman in the Eterna Pokémon Center.
Also, Dr. Footstep will tell you your Pokemon's happiness at Max happiness, he'll give your Pokemon the Footprint Ribbon.