How do I prevent myself from being a crank in areas I know little about?

Limit your opinions to areas where they are desired. Where you have reasonable expertise. And be able to express them such that your audience understands what you’re trying to say and aren’t just bs’ing or talking down to them.

Opinions are two for a quarter, and unsolicited opinions aren’t usually worth that. Life’s short. Don’t be an asshole.

Solid advice in the replies there! My summarized 2c: Don't be delulu. The cranks are often mentally ill, (Especially narcisism), and aren't able to hold their ideas and observations in a consistent reality. Avoid that, and you're cleared hot to be an amateur, and explore fields you're not credentialed in.

Take the scientific method and mindset to heart. Be skeptical. Humble. Your own fiercest critic. What unites the cranks isn't being amateurs; it's screwing up the above.