Use Grade stakes as visual depth indicators.
Larger pours should have intermediate elevation points to guide concrete placement. Divide larger pours into sections of 8 to 12 ft. wide, run a very taught string-line between the forms at those intervals. Drive wooden stakes 3 to 5 ft. apart along the string to become your depth indicators. Position of the string is then marked with a screw or duplex nail partially driven into the stake. A second screw/nail located 1 in. above the first as a backup indicator in the event the first screw/nail is buried. Leave roughly 4 in. of stake above the string so that you can pull it out during the pour. At the section against the wall that has no screed guide or form you may use a bond break that can help keep elevation as well. Using a 1/2-in.-thick asphalt-impregnated fiberboard to the foundation at slab height.