Problem - How can I tell that the spotwelds are good?

Is there a way to do non-destructive testing while the weld is being made?

After a spot weld is made, there is no way other than by destructive testing to ensure 100% quality welds. Understanding the basic principles of resistance welding and keeping track of the major variables will enhance confidence of the resistance welding process. If you want to produce spot welds and want to know immediately if the weld is good, a technique such as "Thermal Force Feedback" (TFF™) may provide this confidence.

The TFF™ feature is protected under the following US patents:

5,789,719
5,504,297
5,254,828
5,111,020
4,419,558