Plot hole: When Andrew wakes up, Anya mentions the First posing as Warren to make him kill Jonathan. Jonathan's body hasn't been found and the only time Andrew became ready to talk, he was knocked unconscious. They shouldn't have a clue about Jonathan's murder.
Get It Done - S7-E15
Plot hole: When Dawn and Buffy discover Chloe hanging in the bathroom, Dawn gives a little gasp but nothing more. But, seconds later, some Potentials run from across the hall saying "What is it? We heard -" before they see Chloe. While it suits the next scene to have them there, there wouldn't have been any sounds loud enough for them to have heard from across the hall through a closed door.
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 ★