Given the following question, let's solve step-by-step. Yes / no, is the following a verifiable fact?
"Plumbing requires extensive knowledge and training in the field to understand what part isn't working and how to fix it."
Plumbing covers everything from toilet pipes to sewer lines, so workers must know a lot about each part and how to fix things. An every day person would not be able to simply walk up and be able to do what they do.
The final answer: yes.