Is there any way of knowing which map's treasure I had already looted e.g. if I had stumbled across it in the course of the game? I want to avoid wasting gold on a map that is useless to me.

I don't think so, unless you check a guide online to see which map corresponds to which treasure.

Thanks. I've come to the conclusion, after some experimentation with buried treasure, it's really not worth it i.e. the cost of buying the map is more than the value I get from selling the contents of the chest and the map.

Also, having one character with high enough Perception will help you find most hidden treasure you walk by close enough.