Instructions
Patronize reputable dealers. You can instantly reduce the chances of encountering a fake by buying from dealers with a positive reputation in the industry. This reputation will be built through years of supplying genuine works of art.
Compare the style and signature of the painting with other known pieces from the same artists. You will soon realize that the masters -- and even lesser known artists -- have their own styles, from brushstrokes to recurring themes. Signatures also can provide clues. Examine all of these things closely and consider whether the painting you are looking at is consistent with other works from the same artist.
Ask for a certificate of authentication. Some galleries will have these certificates for the works of art they are trying to sell. The presence of this certificate will ensure that the oil painting is genuine. The lack of this certificate does not mean it is a fake, but simply that you should be wary.
Feel the canvas which the painting is mounted on and examine the oil paint very closely. Both, particularly the canvas, should have an aged feel if you are buying an old classic. If you are buying a newer painting and you want to ensure it is a genuine oil painting, smell the canvas. Oil paintings retain an oily smell for many years until the oil fully dries.
Study the market of oil paintings similar to the one you are considering and from the same artist. The painting you are looking to buy should be priced appropriately. If it seems too good to be true then it probably is; be wary of dealers selling "precious art" for knockdown price.