It's hard to say for sure if a collection of found buttons are all from the same uniform coat, unless they are found on fragments of a jacket and in a tight group. You might find some buttons in a very defined area, but that could be because a coat was lost or discarded, or they might have been lost in a pattern around a long gone fox hole or around an old well in the yard of a Civil War era cabin. Let's say you are dealing with a standard five button frock coat, it is also possible that the coat was missing a button , or had a button lost and replaced with a slightly different button with a different backmark, so that when you find them there are slight differences even though they would be from the same coat.
When I was a kid I used to obsessively hunt a Civil War- Indian Wars period fort site. There were a few occasions where I found general service eagle buttons numbering four or five in total and in a small area measuring maybe no more than a couple of feet or a few feet in diameter. On those occasions there would be four or five, which would correlate to a common issue coat. I personally felt that there was a good chance that these finds represented lost or discarded coats, but really there is no way to be certain. It remains at most a possibility, and that conclusion has to be good enough.