Some Irish stone rows are thought by some to be the remains of old walls. It was once the practice (apparently) to build dry stone walls in sections with orthostatic stones every so often. I have never seen an intact wall of this construction, so I can not take it as gospel.

Pairs of evenly matched stones in a field could indicate an old gateway, where the hedge has been taken up and the stones left for rubbing.

Position is also key ...

If a stone is in the centre of a small field then it's likely to be a scratching post. In the middle of a large field it could have once stood in a hedge and been a boundary marker.

On a ridge or hilltop it is also likely to be a boundary marker.

Alongside a track/road it's likely to be a route marker.

I also consider landscape features and a stone's relationship to them.