One of my grandfather's was a plumber. From what I understand, it's a pretty good field to go into. I know by the time he and my grandmother were retired they had a brand new home that was paid in full and two cars that were paid off also.
Anyway, I'm not sure what the testing was like when he started out as a plumber 60 or so years ago. However, I had to take a state-run test to get my insurance license, so I can offer a few tips.
First of all, as the article said, take classes that are from or approved by the organization that is giving the test. This seems like a no-brainer, but it needs to be said.
Second of all, if the organization that administers the test sells a study guide, buy it! This was invaluable to me when I took my insurance license exam. I already had a pretty good idea of how the test was formatted and what the questions would be like. It made me feel much more comfortable.