How can I make sure it's legal to "help" in return for lodging on a non-working visa somewhere? [closed]

While I appreciate Zach's detailed answer, I'm going to add a little more that I've recently learned:

What Zach quoted shows me that WWOOF-USA knows that they are helping people to break the law and explicitly advises them to lie about it. From looking around the web, I find complaints suggesting that they have abuses in other countries. One report ("he said/she said") alleges that WWOOF deliberately removed the reviews that would warn others about a particularly egregious violation of law (but by the host; the volunteer was the alleged victim).

WorkAway.info is more honest—or at least smarter. They warn in their T&C that you need to make sure you get the appropriate visas before going.

Finally, on the China question, another TSE post mentions a need to register each place you stay there. That suggests that either the authorities tolerate the "work away" arrangements, or if not, the registration would quickly result in the very things I want to avoid.