A dowser would use information dowsing, posing questions to the pendulum/ L-rod.
We would need something to start with though, like the name of the battle, who was involved (Generals,ect.) and a time period when it started and when it ended would help pin it down.
Some secondary information would be; was the battle fought near a creek/river/mountain/town that is still there or still called the same name today as it was then.
You could try to have a dowser in your area assist you and have the person go with you into the field. Either way would work, field first or map work first then field check. I prefer map work first then field check.
Jon