As is the case with any antique lighting fixture, it is highly recommended that you look for signs or markings that helps you identify the manufacturer and year. In case of bridge lamps, the markings were mostly stamped on the base or under it.
The older the lamp, the more difficult it will be to find the markings because it can get faded or difficult to read due to the patina.
There are many online public resources that help you identify antique lamps and ascertain their monetary value. All you need to do is click some pictures of the lamp and upload them to these websites.