undercut gagePlace this V-Wac gage beside the weld and use the little pointer, place the pointer in the undercut or low area along the toe of the weld and read what the gage says on the scale end of the gage.
Here's instructions for using a fillet weld gage:
check the weld from two directions(vert/horz) to make sure the legs are sufficient size(don't try to go by the little lines that are scribed on the gage, check it in both directions), and the weld has enough throat by using the double arc looking gage.