Freelance English Language Work

[Thaiquila]

I have read that many English teachers do sideline jobs such as private tutoring, casual group conversation, writing, editing, and the like. I assume these casual jobs are paid in cash and that this sort of practice is quite widespread. I am wondering about the legal ins and outs of these activities, relating to visas, work permits, and taxation.

Could a person perform these activities without a work permit or teaching certificate? Assuming it is not really legal, what are the chances of getting caught and what are the consequences? In other words, is this a priority enforcement area for immigration?