An expert will provide you with an estimate, a term that accurately describes the speculative aspect of appraisals. Since each work of art is unique, experts will search for comparable works that have recently been sold at public auction (or ones they know of on the private market). Their experience makes they uniquely capable of interpreting sale prices available from online databases.
Every work of art has two values. In the same way that every financial product has a bid price and an offer price, in the art world we make a distinction between the realisable value and the acquisition value. The gap between the amount of money asked for a work of art, and the amount that this same work of art would bring in if you were to sell it is the difference between the work's offer and bid prices – the profit margin in the art trade.
Do you need an appraisal for insurance purposes, or for reasons having to do with a division of assets or a succession? Or are you contemplating a major purchase? The services of an expert are the only way to obtain reliable, confidential information adapted to your situation.