Storyteller - S7-E16
Visible crew/equipment: During Andrew's fantasy of him, Warren and Jonathan being Gods, something dark is reflected in of the piles of gold. Possibly a camera or camera crew.
Visible crew/equipment: When Dawn and the Potentials try to get past one of the police officers at The Bronze, Kennedy says that he'll have to shoot them all to stop them. As he responds with "It doesn't really bother me", the shadow of a boom mic can be seen on the wall behind him in the top right hand corner of the screen.
Visible crew/equipment: After Buffy has spoken with Clem and is walking down the road, you can see the reflection of a camera in the back of a green SUV.
End Of Days - S7-E21
Visible crew/equipment: Right at the end, when Angel saves Buffy from Caleb, the shadow of a boom mic can be seen in the top left corner of the screen as Angel helps Buffy to her feet and says, "I was never much for preachers."
Visible crew/equipment: When the 4 Scoobs meet in the hallway for the revolving shot, just as the camera starts to pan over you can see someone casually walk down the hall into frame behind Xander's left shoulder, as the camera starts to revolve you can the person is now crouching in the shadow behind Giles. It appears to be a crew member that accidentally walked into shot at the wrong moment and hid quickly.
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 ★