How would they even know? Assuming they keep quiet about it
sometimes licenses are different between working for yourself vs someone else. Other times your employer can't take on jobs without you and your license present.
Then there's insurance differences between self-employed and employed ______ tradesperson.
> Get on Yelp, ask his customers to leave good reviews, and build a business.
You can’t do a good business without leaving visible traces - permits, customers, suppliers, etc. all provide ways for information to leak. If his employer is exploitative, he has above average odds of correctly fearing retribution and depending on the area that could mean unemployment. It’s not exactly uncommon for companies to discretely agree not to hire certain people.
This is the key reason why for most jobs, small town America is flyover country, an economically inefficient, exploitative, broken system. Without a large labor & employment market within easy commuting distance, your skills aren't portable if you get fired, and your boss can't readily find a replacement, so most employment relationships end up in a reluctant, often abusive state from one or the other.
From other specialist non-trade business I'm familiar with, people are normally aware of new employees coming to town before they even start working. People talk to each other.