Unsigned paintings are notoriously difficult to attribute. If your item has no provenance, and you do not even know who the famous painter is likely to be, then I'm afraid you may have a tough time proving it to be valuable. It might be worth sending an email query to one of the larger auction houses where there may be experts on European art of the correct period. If you do go down this route, be sure to include good photographs of the painting from both front and back, including close-ups of any framer's labels or gallery labels. Give as full an account of the item's history as you are able.