Your best bet is to contact an auction house that specializes in antiques and/or art. Try local houses first. Many auction houses will offer a free evaluation of even an unsigned painting, especially if it is high quality or interesting painting in a good frame. Estimates are provided for works that they think will sell. They will not be interested in low quality work.
Contact the auction house via email. Attach a high-quality photo of your painting.
You can also contact local art dealers. If they are interested, they may offer some information that will help you. If, indeed, the painting appears very interesting to a dealer, take it to several. Remember that a dealer can offer you a tiny fraction of what it is worth in order to make a hefty profit.