As the Supaguard treatment was applied six years ago it's likely the sealant has since dissolved. However, you can check by pouring water over a horizontal panel (bonnet, bootlid or roof) and seeing if the water beads tightly and rolls off quickly or not. If it does bead there is still a fair amount of sealant left.

We would recommend purchasing a clay bar to remove old wax/sealant as it's the most effective method. Wash the car thoroughly prior to using the clay bar, however. It's beneficial to do this before applying a new ceramic product.