Look at the clouds. The types of clouds in the sky, as well as the direction in which they're moving, can potentially tell you a lot about upcoming weather.
Clouds going in different directions (e.g. one layer going west, another layer going north) means bad weather is coming.
Cumulonimbus clouds early in the day and developing throughout the day can mean greater chances of severe weather.
Check the grass for dew at sunrise. If the grass is dry, this indicates clouds or strong breezes, which can mean rain. If there's dew, it probably won't rain that day. However, if it rained during the night, this method will not be reliable.
Look for a red sky. Remember the rhyme: "Red sky at night, sailor's delight; Red sky at morning, sailors take warning." Look for any sign of red in the sky (not a red sun); it will not be a bold orange or red the majority of the time, but that depends a little on where you live.
Look for a rainbow in the west. This is the result of the rising sun's morning rays from the east striking moisture in the west. Most major storm fronts in the Northern Hemisphere travel west to east, and a rainbow in the west means moisture, which can mean rain is on its way. On the other hand, a rainbow in the east around sunset means that the rain is on its way out and you can look forward to sunny days. Remember: Rainbow in the morning, need for a warning.
Observe animals. They are more likely to react to changes in air pressure than we are.
If birds are flying high in the sky, there will probably be fair weather. (Falling air pressure caused by an imminent storm causes discomfort in birds' ears, so they fly low to alleviate it. Large numbers of birds roosting on power lines indicates swiftly falling air pressure.)
Seagulls tend to stop flying and take refuge at the coast if a storm is coming.
Animals, especially birds, get very quiet immediately before it rains.
Cows will typically lie down before a thunderstorm. They also tend to stay close together if bad weather's on the way.
Ants build their hills with very steep sides just before a rain.
Cats tend to clean behind their ears before rain.
Turtles often search for higher ground when a large amount of rain is expected. You will often see them in the road during this period (1 to 2 days before the rain).
A very old wives tale says if birds feed in a storm it will rain for a long time, if they don't it will clear soon.
If you see a red sky during sunset (when you're looking to the west), there is a high pressure system with dry air that is stirring dust particles in the air, causing [[the sky to look red. Since prevailing front movements and jet streams weather usually move from west to east (see Tips), the dry air is heading towards you. A red sky in the morning (in the East, where the sun rises) means that the dry air has already moved past you, and what follows behind it (on its way towards you) is a low pressure system that carries moisture].