The biggest ones, often requiring identification too, are mostly legal.
The vast majority of the smaller ones definitely aren't though and with a good reason: It's way too much work for a small casino!

Anyways, like others have said, it differs per country. Not only your's, but also the country the casino is being operated from. I'd say it doesn't really matter though.
You can probably trust the bigger ones that they got the right papers and the smaller ones most likely won't be shut down by the government either.

Anyways, back to your question: Big ones, a lot. Small ones, almost none.