There's no easy solution to this I'm afraid. You might try comparing the signature to those of listed artists either on one of the many artist signature sites (you will probably have to pay a subscription to use these) or else use an artist's signature reference book. These are widely available on Amazon, eBay, etc., and can sometimes be found in public libraries. Please be aware that many amazing artists never become listed, simply because there are many talented amateurs who never produce a large enough body of work to attract mainstream attention. This is great news for those who collect art for the love of it, but not such good news for those hoping to turn a profit.