Take notes on the specific style of your painting, along with the basic characteristics of the painting. Determine which way the brushstrokes are pointing, whether the painting is abstract or realistic, and the subject matter and setting of the painting. Also, note the colors used in the painting as specifically as possible. For example, if the color is burnt orange, don't write simply "orange." Some paint colors were not available in certain time periods, so this may help determine the age of your painting.
Check the painting carefully, using a magnifying glass if necessary, to find any distinguishing markings on the painting itself. The painter's signature would be the best type of marking to find, but other markings, such as a stamp or branding on the frame or writing on the back of the canvas or paper, would be significant. Take detailed notes about your findings.
Search online for any information about the markings you've found, such as business owners, frame builders, or the painter. Take note of any information you find that will help determine the age of the painting and the identity of the artist. Note that some collectors will pay more for something with certain brands or symbols that hold clues to its age.
Search general online auction sites or art-related auction sites using the distinguishing markings you've found. Try to find paintings for auction (or that have sold at auction previously) from the same painter, of the same era or with the same general characteristics. Take note of the prices that these paintings went for at auction and what some have sold for in the past to get a general idea of how much the painting is worth.
Meet with an art appraiser. Note that professional art appraisers have education and background knowledge of art and will most likely be able to tell you how much your painting is worth or how much it would sell for at auction. Tell the appraiser the painting details and the online auction information you have discovered. Have the appraiser do his or her own research and look at the details of the painting so they can properly asses how much the painting is worth.
Take your painting or send a high-quality photograph to an auction house so that the staff members can share their thoughts on how the painting would do at auction. Inform the auction-house staff members of your findings and what the art appraiser has said so that they know all the details before determining how much the painting is worth.
Using the professionals' input and your research results, determine how much your painting is worth if you plan to insure it or put it up for auction. Note that the worth should represent what those sources have said. If you plan to insure the art, insurance companies may want an official appraisal.