I'm trying to get an Umbreon and the only way I know to check happiness is impossible, so how do you do it?
In X & Y, it's given by a girl in Laverre City.
There's a place in Laverre City, its the Pokemon Fan Club of Kalos. Before going up the steps to the Poke Ball Factory, make a left, and its the second one in. There's a lady in front of the stove. Talk to her.
There is no other way to check your pokemon's happiness without continuing through the story line till you get to Laverre city, then you will be able to check your pokemon's happiness.
Actually, there is a way to know whether you have maxed out the happiness of a Pokémon or not in X before you get the 2nd badge. If you know your berries, you can feed your Pokémon with any of Pomeg, Kelpsy, Qualot, Hondew, Grepa and Tamato. However, each of these will lower a stat of the Pokémon you are feeding so if you don't want to lower a particular stat, it will take some time and lots of patience to grow and harvest those berries.
Those berries can be created by planting two berries next to each other if there is a mutation:
Pomeg Iapapa + Mago Kelpsy Chesto + Persim Qualot Oran + Pecha Hondew Aspear + Leppa Grepa Aguav + Figy Tamato Sitrus + Lum
If the happiness of the Pokémon is maxed out, the berry won't have any effect.
These particular berries actually have 4 possible messages, let's take Eevee and a Pomeg Berry as example:
The stat is reduced and happiness is increased:
Eevee became more friendly! Its base HP fell!
The stat cannot be reduced but happiness increases,
Eevee became more friendly! Its base HP can't go lower!
The stat further decreases with no other additional effects (Max Happiness)
Eevee adores you! Its base HP fell!
Neither the stat can be decreased, nor happiness can be increased, that's when it won't have any effect. (Max Happiness)
Eevee adores you! Its base HP can't go lower!
Otherwise, if you don't want to actually use the berries, you can save your game, use the berries and count how many you used and then load your previous game. The more you used, the further your Pokémon is from max happiness