The only way to test is to pop a tile off and view the back which isn't very practical for a wall project .... too much damage.

80% is what a properly set tile should have for walls. Each tile should be set into place when the mortar is wet and fresh on the wall.
When a tile is set, it should be twisted into place so that the mortar is "mooshed" around a bit between the 2 surfaces. Just setting the tile into the mortar will give you about 50% coverage if you're lucky .... a couple of twists does the rest.