To determine a painting's value, a good place to start is to find auction records for other paintings by the same artist. The value of an oil painting is affected by several factors including its age, condition and the artist who painted it.
Major auction house such as Sotheby's or Christie's list records of previous auction sales online. For example, Sotheby's lists auction results as well as holding a sold lot archive. If the artist who made the oil painting is known, it should be possible to find a previous sale of one of their works. It may also be worthwhile to contact an auction house directly, as they should have an understanding of the current market for the painting.
The value of an art piece can also be affected by its condition and age. If a painting has cracks or water damage, it may be worth less than a better-preserved work. However, the age of the painting may excuse a certain amount of wear. A painting that is 100 years old can be expected to have a few minor cracks or discolorations. In the case of an oil painting, it is best to leave the original frame on the piece if possible.