Reverse painting was created by painting an image on glass then reversing it for framing. It is a very old practice that has changed over the years. Look at your piece through a magnifying glass. If there are irregularities like bubbles or a slightly wavy surface, it was probably made before 1903 or so.
Keep your reverse painting out of direct sunlight and away from any heat source like a heat vent or fireplace.
Values are all over the place. If there is damage like cracks, faded or peeling paint, the piece will have little value.
You may find information by visiting the Corning Museum of Glass site. If the piece is a silhouette, check out the book "Vintage Silhouettes on Glass and Reverse Painting" by Shirley R. More.
If you are still unable to find out anything, take it to a dealer or an appraiser. You will have to pay a fee. Ask about the fee before you visit.