For electricians and welders, there are licenses / certification. Obtaining the license / certification demonstrates the baseline competency
Isn't this like a college degree?
Work history, references, general questions and maybe some whiteboarding. Honestly just a regular interview.
Software development is certainly more creative than electrical or welding work, and thus certifications are more meaningful. In software engineering, what certifications you have or what languages you're familiar with is a comparably poor predictor of your success on a given project.