Continuity mistake: In 'Flooded', Jonathon reads a few things off the board listing the Trio's ultimate aims. One of the things he reads out is "prototype jet-packs that actually fly", but when the list is shown a few seconds later jet packs aren't mentioned on it. (00:27:10)
Revealing mistake: Near the end, the TV behind Andrew shows the periscope's view, but it doesn't match the movements he makes with it. (00:36:25)
Continuity mistake: As the Trio are discussing their plans near the end of the episode, Jonathan's arms are crossed in closeups and in other positions in wide shots. (00:36:35)
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 ★