Although no longer common outside of the luxury market, bonnet ornaments have a long history. Originally built to obscure radiator caps, bonnet ornaments became prominent symbols of the manufacturer and the model. Theft was often a problem, and combined with some danger to pedestrians in a collision, this has made bonnet ornaments somewhat rare. Identifying a car by its bonnet ornament requires research. Although many newer bonnet ornaments incorporate the brand name or logo, older ones were often unique to the car model. With enough time, it should be possible to identify any bonnet ornament through enthusiasts' websites.
Identify what type of ornament the car has. It may either be a logo-based ornament, such as the famous blue-and-white BMW symbol, or a "mascot," a sculpture or figure common on older cars.
Learn whatever you can about the car's history. If you know what country the car came from, or roughly what year, that can narrow your search significantly. Most cars with mascots come from 1950 or earlier, while later-model cars are likely to have logos. Some details can also tell you whether the car was European or American, such as the shape of the license-plate area, if there is one. European plates are longer and narrower than American ones.
Compare your ornament or photographs to those in online databases. For logo ornaments, CarType has a well-organised database, broken down by brand. For mascot-type ornaments, the place to look is Mascot Mania. A Google Images search for "Hood Ornament Identification Guide" will bring up a series of posters showing historical bonnet ornaments for most American makes.
Send an e-mail with a picture of your bonnet ornament to the administrators of car enthusiast websites. If you haven't been able to find any photos of your ornament through your own research, these people may be willing to help.