It ain't simple so I always just rely on the geo's report in the maiden resource or the subsequent resource updates. Essentially the drill assays create a geological model that then can be used to estimate the resource size and purity. In Australia we have the JORC code which has a set of required procedures and methods and two levels of resource confidence ; indicated and inferred which added together give you the total resource tonnage. That means that all JORC compliant estimates are comparable. Here are some useful sites:
