Determine the approximate age of the painting. Often, older paintings have more value than newer ones, especially if the painter has passed away. Some painters put the date of completion next to their signatures or on the backs of paintings. If there is no date, the condition of the canvas can give you some clues. Older paintings often have uneven, rough-looking threads.
Assess the painting's overall condition. Even if a fairly well-known or popular artist painted it, it won't have nearly as much value if it's not in good condition. The most valuable paintings are free of cracks and tears and have not undergone touch-up jobs. If any of those things are present, they will bring the value of your painting down somewhat.
Research the value of similar paintings. Check online art auction sites to see how much paintings by the same artist, in a similar condition to your own, are currently bringing in. A painting can be valuable on paper, but its true worth lies in what others are willing to pay for it on the open market.
Have your painting appraised by a professional appraiser, preferably one who is a member of the Appraiser Association of America. When he gives you the amount, ask how he determined it. If it doesn't seem to match up with your own assessment, ask specific questions. For example, if the appraiser says that your painting's condition is bringing its value down but you know that it's free of cracks and other flaws, ask him what is affecting the condition negatively. If his estimate is below recent sales of similar paintings, ask him why his appraisal is coming in lower.
Get a second opinion from another professional appraiser. If it comes in close to the amount the first appraiser gave, you probably have a good idea of your valuable painting's true worth. If there is a big gap between the two appraisals, consult a third professional and compare to see which of the first two is closer to reality.