As you cut through the part, take a hack saw blade and grind off all the teeth so you have a thin long gauge that will fit down the kerf that has been removed. Let’s say the material is 6” thick. Put a mark on the blade at say 6.5”. Then take the blade and insert it into the cut and with the bottom of the blade sticking out the bottom, slide the blade along the tool path and check that the material is cut all the way through. You of course have to make allowance for grates. But this will give you a good guide as to the status of the bottom of the cut.