Dew on the Grass
When morning dew forms on grass, it means the sky was mostly clear the previous night, the earth cooled and temperatures fell. These cooler temperatures cause water to condense, creating dew (or frost in cooler weather).
If the grass is dry in the morning, it means clouds could have kept temperatures at or above the dewpoint and could indicate rain is on the way.
The method obviously isn't reliable if it rained overnight.