Continuity mistake: When Veruca comes to Oz's cage to spend the night there, she tosses her jacket onto the round stone thingie (knob?) at the end of the stairs, and it covers the whole thing. However, when Willow discovers them in the morning and Veruca goes to pick up her jacket, you can see that it has moved. Willow didn't touch it when she entered, and both Oz and Veruca were locked up so they couldn't get anywhere near it. (00:26:35 - 00:29:20)
Continuity mistake: When Oz leaves and holds Willow close to him, the position of their noses changes depending on the camera angle. In the shots of OZ they're pressed together and in the shots of Willow they're not.
Continuity mistake: When we see Veruca eating at the table, she reaches for a cup as we see Oz coming up behind her. The shot changes and we see her turning around and reaching for the same cup again, though she never had time to put it down.
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 ★