They could probably tell fairly quickly by the age of the ground they were found in. Colonisation occurred less than 200 years ago making it fairly trivial to understand if they were older. The indigenous had also maintained stories describing the area as a burial ground.

For the living indigenous its a tangible link to their pre colonisation culture, thus making it incredibly important to them. After they’ve had so much of their land, language, beliefs, foods and culture has been taken away from them, I’m sure you can understand why preserving the links that they still have is important to them