The best method is looking at Copryright laws and Fair Use yourself. There's some changes that have been made recently that I haven't delved into very much and how it affects Fair Use. However, Fair Use does apply even to the selling of fan art.
I would suggest, however, to avoid fan art without written consent of works from companies that have a litigious history.
Be careful of what you're watching because it can be misleading or just wrong. In this case, he's definitely one of the most informative out there. I still argue your best bet is due diligence of your own. Any website that includes the disclaimer "I'm not an attorney," don't rely on it too strongly. You want the interpretation from a lawyer, or the written law itself.
Do also keep in mind that this is one of those things that are typically not enforced. You will not be criminally persecuted for copyright infringement but it is something an IP owner can sue if they wish, provided your work isn't protected under Fair Use.
TLDR: if you're someone who wants to sell fan art on an official platform and expect a decent following, you want to do your due diligence on what is permissible.