A big part of knowing what to wear is knowing what the weather will be like. I've found this to be the best site for weather forecasts.

Click on wundermaps in the upper right hand corner so you can see where the weather station is. In mountainous areas, it's usually in a nearby town at low altitude. I usually pack one layer more than I need for the expected temperature unless it's a short hike. Historical weather is also available on this site making it easy to plan trips in the future.

If you are going on anything but tourist trails, I suggest leaving a good itinerary with a friend. I use email for this. Include pictures of yourself wearing the clothes you will be wearing. Also include a picture of your car showing the license plate. I use email because it's easy to forward to search and rescue if needed.