I was slow and not particularly mature and didn't get an inkling until I was 19, and even then it took me a couple years to figure it out and finally come out. Absolutely no signs beforehand, everyone was surprised, myself included. Some people have always known, others come out when they're 14, others switch back and forth. For some it's obvious, but that's not always the case. Probably depends on one's situation too - my friends were all straight girls who weren't particularly interested in relationships, so it was never a big part of my life and I never really needed to think about it until I actually fell in love with a girl.
What you can do is model acceptance and openness, not just towards gay couples but as a general attitude. Keep the communication lines open and listen, and if it's there, it'll come out eventually (sorry, bad pun

)!