Underexposure is commonly characterized by 'foggy' looking images with poor contrast. The whites commonly appear grayish, lacking detail, and the blacks and darker colors appear light and foggy.
Overexposure is commonly characterized by overly, high contrast in the image, with 'burned up' whites and light colors.
Flashed out overexposure is characterized by 'burned up' faces and subjects in your images when using a flash attachment. While they are typically overexposed images, they can often be corrected in printing, when you use a good film processor.
There can be many causes for overexposure and underexposure. Several common possibilities are: