There are three questions you can ask yourself when taking photographs that will help you determine whether or not you should use fill flash on that image.
- Is my subject in a shadow?
- Is there more light behind my subject than in front of it?
- Am I close enough for flash?
If there more light behind your subject than in front of it, your image is "backlit". Even though your eye may be able to read the scene very well, remember that the camera does not record light the same way you do. If there is a lot of light behind a subject, the camera will almost always underexpose your image - even if you meter off of the subject itself.
If the answer to either of the first two questions is "yes", then consider the third question. Even in situations where your photograph would benefit from fill flash, if you are not close enough to use a flash it is a moot point. The flash that is built into your camera is usually only powerful enough to light a subject about 9 feet away from you indoors. This distance is further compromised when outdoors and in brighter situations.