Other mistake: When Angel fights with Buffy in the forest you can see that he hasn't got shoes on, just socks. Later, when Buffy goes back to the mansion and Angel is chained up, he's wearing shoes. How did he manage to put them on, if he was in chains?
Continuity mistake: When Buffy is in the counsellor's office she is wearing a yellow kind of gauzey cardigan which alternates between being completely buttoned up to unbuttoned between shots.
Revealing mistake: In the teaser of "Beauty And The Beasts", when Jeff is running through the woods, you can see in the top left corner of several shots a lamp being used to light the scene.
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 ★