There are certain tell tale signs that help a float plane pilot to determine the speed of the wind.
A wind of 1 to 2 knots: smoke drifts and water ripples.
A wind of 4 to 6 knots: leaves rustle; small wavelets break the surface of water bodies but there are no whitecaps.
A wind of 7 to 10 knots: leaves and small twigs move as do flags; scattered whitecaps break the surface of a water body.
A wind of 11 to 16 knots: small branches move; long waves and frequent whitecaps can be seen on water surfaces.
A wind of 17 to 21 knots: small trees sway; many long whitecaps break the water surface.
A strong wind of 25 to 31 knots; large branches move; large waves, spray and white foam crests break the surface of water bodies.