Look on the image of the artwork you want to find the title of and see if there is an artist's signature anywhere on it. On paintings, the signature is usually on a bottom corner, and sculptors usually sign the base of the artwork.
Go to a Google search window and click on the word "images" on the upper left hand corner. If you have the artist's name of the artwork, type it in the search window and see if you can find an image match. If you click on the image and let the website load, usually the title and any relevant information can be found easily.
Find a time period for the artwork by searching "art movements" and try to match your artwork with an art historical movement. If a work of art is Abstract Expressionist, it will narrow your search down to artwork primarily created from the 1940s to 1960s.
Email a professor of art history or a curator at a museum. Most professors have email addresses listed in the faculty section of the school's website and may be able to help faster than searching online. Curators can usually be contacted through the museum's website.
Visit an art store with the image of the untitled artwork. Many people working in art stores are art students or people who collect art. They may know the title right away.
If all else fails, take a field trip to a local museum and bring your image with you. Curators are trained to be able to look at an image and tell you what style it is and perhaps the influences behind the artwork, helping to narrow your search. Museums usually also have libraries that can be accessed with help of a curator.
Call an auction house. Sotheby's or Christie's have auction houses around the world. They are staffed by highly qualified professionals who specialize in artwork from all time periods. If you don't have any further information than the image and no one else can help you, they may be able give you details you are looking for.