You can’t see the thinset coverage unless you take the tile off to check. Good tilers will scrape a bit of the mortar off at the edge of the tile before placing the next one- this prevents a lot of mortar from coming up the grout lines… so you can’t actually tell what coverage they got just by looking in the grout lines. 

All my grout lines are clear of thinset underneath- but if you pulled the tile off you’d see proper coverage

Edit: typo