You can often tell when a rain storm is about to come in the woods by a gentle breeze that mostly touches the tops of the trees. Often there will be little breeze at the ground level but you can hear the trees blowing in the wind.
The sky gets darker, you might even feel a few big drops of water. In the summer, if it’s a thunderstorm you might hear a clap or two of thunder. Then sometimes it gently ramps up, but usually it comes down strong, quickly dropping quite a bit of water on your head. You seek shelter as soon you know you will get soaked.