How would you go about finding out for sure? Do you need to visit the foreign embassy? Or is there some kind of lawyer you would talk to? If so, which kind?

As a non-American working (legally) for a while in America (in Silicon Valley), I ran into a situation where my visa lapsed and I wasn't sure where I stood legally (like I wasn't sure if I was even allowed to be in the country any more). I can't remember how I found a lawyer. Anyway, I did a 15-minute consult over the phone. Actually, he was really casual about payment, at the end he said "mail a cheque to my secretary" and hung up, haha. Probably was mostly treating the call as a loss leader.

Anyway, among other things, he said that doing any work inside US borders, if not on a work visa, is technically illegal. Like apparently if I'm on vacation in the US, and I pull out my phone to answer a few work emails, that's technically a visa violation, not that in that case anyone's gonna mind. IANAL TINLA. And of course you're interesting in jurisdictions other than the US.

So, yeah, I think the way to find out is find a lawyer in the jurisdiction you'd be wanting to work from, one who advertises themselves as specializing in visa issues.

I looked up the relevant tax codes myself and got them vetted by my accountant, but I'll admit it was a really clear cut situation for me and my government has really well written descriptions of the codes online.