Dirty Girls - S7-E18
Xander: I've been through more battles with Buffy than you all can ever imagine. She's stopped everything that's ever come up against her. She's laid down her life - literally - to protect the people around her. This girl has died *two* times, and she's still standing. You're scared? That's smart. You've got questions? You should. But you doubt her motives, you think Buffy's all about the kill, then you take the little bus to battle. I've seen her heart - and this time not literally - and I'm telling you right now, she cares more about your lives than you will ever know. You gotta trust her. She's earned it.
[Pause.]
Faith: Damn. I never knew you were that cool.
Buffy: Well, you always were a little slow.
Chosen answer: From what we see in the series, certain girls are identifiable as potential Slayers - Kendra clearly fell into this category. The identification method is presumably mystical in nature, but the Watchers' Council are pretty effective at that sort of thing, so they're quite good at tracking down the potentials ahead of time. Not perfect, though - it does appear that Buffy herself may have slipped through the net - certainly she had no inkling of what she was until she'd already taken on the role of Slayer. It is possible, however, that this was actually cultural - an American family would hardly be likely to turn over their daughter to some strange man for 'training', so the Watchers might have chosen to keep an eye on her covertly, whereas some other cultures (like Kendra's Jamaican parents) might be more willing to believe.
Tailkinker ★