A good quality spot weld usually follows a classical dynamic resistance curve as shown to the left (yellow line). When welding current is applied (as shown in the red line), the resistance across the electrodes will typically show an increasing resistance. This will peak at the point where the molten metal of the weld nugget will allow the penetration of electrodes and thus show a gradual decline of resistance.

A tear down of this spot weld will likely measure a nugget size of acceptable quality.