Is there a certification or license required for visa agents in Thailand?

Tour guides yes, visa agents no. Many businesses that use them find and hire them by word of mouth, from business friends that also use them for their foreign employees, or for themselves. The Thai/(your home country) Chamber of Commerce can also help you find the right one.

Nope not even for real-estate agents either.

No, there is no licensing or certificatioms for visa agents.

I see where your coming from as i too thought about this. All i can suggest is look up i dependant reviews ie trustpilot, google, trip advisor. Good luck