How do know if bands like Slip Knot is singing when they wear masks and their lips are sewn shut ?

i just went to a slipknot concert on valentines day, and their lips aren't sewn shut during a concert, watch the music video dead memories by slipknot and you can get an up clost of the lead singers mask, there's a large opening for his mouth. and the band HU they don'y wear their full masks during concerts