How can I tell what time it is in Pokemon X/Y?
Just press home. That will take you back to the main 3DS menu, and there's a clock in the upper-right corner of the top screen. There's no in-game way to see the time, but the in-game time is identical to the time on your 3DS.
Actually, there is sort of a way to tell the time in game. Just press save and it will bring up the save screen, along with a time stamp. This time stamp corresponds to your 3DS time stamp (although, in 24 hour format I think).
Then cancel the save, and continue becoming a pokemon master!