for people who love the littlest dairy goats
If he's never bred a doe before, this is not that unusual. It's good that she's young. Older does can really beat up on young bucks. I'd do exactly what you've done in this case -- leave them together and hope he figures it out. And mark you calendar for 145 days.
You can't tell by looking if she was bred, especially if they were together overnight. Definitely mark the calendar. I'll never forget the woman who bought goats from me and kept calling and emailing saying that they were never coming into heat. Then one day she called me in a panic because they were both in labor! Turns out she left them together for 15 minutes and went into the house one time five months earlier. The way goats breed, it is not an exaggeration to say that you'll miss it if you blink. It only takes seconds.
I would not expect to see anything the next morning. If you were watching and he bred her two or more times while you were watching, then you might see some semen leaking out, but I wouldn't expect to see anything even half an hour later. They don't have that much semen.