Primarily you need to do research on the routes of march to make sure you're not in the wrong area and it really helps to get a period map that will give you an idea of how the road(s) have changed. It possible to be a mere 100 feet from what you're looking for because the road isn't where it used to be. Other than that, you need to know what kind of commander was in charge of the unit that made camp where you're looking. If he was a West Pointer he would probably have followed the standard strategic camp rules of putting the camp in a defensible position that wouldn't be easily surprised, yet would have fairly easy access to water and wood. If he was a political appointee he might have had his unit make their camp anywhere. It really does help to know who's camp you're looking for.
Bamabill's advice is good. Research is the key. Armies stuck mostly to the roads and almost always camped near water.
Maps of the period are necessary. Many are available for free on the internet. You'll be surprised how many roads existed back then that are all but gone now. Same with the creeks and streams - especially if you're in an area where modern development has changed the topography.
I live in a suburban area where there's not much open space left. I got permission to hunt a 1 - 2 acre field with an 1880's house on it. Talking to the owner, she said there used to be a creek before the modern houses were built in the 1970's. There is NO sign of it now. Sure enough, I found bullets and buttons and horse tack - even an arrow head - on the lot.