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Drop a small spoonful of starter into a glass of room-temperature water. If it floats, there's enough trapped gas to raise a loaf. If it sinks, it usually needs more time.
The test isn't infallible. Stiff starters can sink while perfectly active, and a stirred sample loses its bubbles before it hits the water — scoop gently from the top.
Pair the float test with the signals that never lie: a doubling in volume, a domed and just-beginning-to-recede surface, and a smell that's yeasty and sweet rather than sharply acidic.
