A good Question! Whatever the medium, skill level is the first criteria. Are the materials used high quality. Is the work on proper canvas, paper etc for the medium used. What about the image. One might find sketches on less than quality paper, but by an artist of note. Don't pass it by. An educated developed eye begins to see schlock for what it is. This comes under the category of if the buyer /observer looks at enough work he/she knows. Certainly there is work that may not appeal to the observer, but it is still very good work. Museums and galleries can offer visual information as to what is considered high level and talented work. There will be times when there is disagreement, but in general, museums are a good place to educate the eye. Again books showing work in prominent museums can help us understand. Take a class at a community college or offered through museums and galleries. Learn to understand perspective, color, dynamics of a piece. Does it move? Does it move the observer, does it take you home or to a magical place.
"Art washes away from the soul the dust of everyday life".