Is usually covered in either the site's privacy policy, or their TOS.
"can I be held responsible for comments users make on my website's forum?"
In many cases, no; but not all of them.
Overall, to answer your question I'm sure a business attorney would be willing to answer such questions without any cost. I've consulted with a few of them, drop in schedule a meeting and just ask some general questions without any sort of consulting fee that you'd get with people who call themselves 'specialists'. With this sort of thing, I'd go directly to a lawyer; you avoid both bs fees, and also getting involved with someone offering legal advice without a license which can land you both in trouble.
Of course you'll probably also be sued.