Never Kill A Boy On The First Date - S1-E5
Visible crew/equipment: When Buffy and Giles are in the cemetery, right before Buffy bails to go meet Owen, you can blatantly see a number rectangular studio/set lights reflected in Giles' glasses coming from various angles. (There wouldn't be dramatically-angled lights like that in a cemetery, let alone after dark).






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 ★