The biggest error people make is listing the following:
from flat earth, creationism, the food babe, fad diets, anti-vax healing crystals and energy channeling
and saying 'so how do I tell that it's pseudo-science?'
This alone shows you are in a wrong mentality to handle the deduction of pseudoscience. You must come in knowing nothing, believing nothing. Every single Round Earther may be a sheep or in on the conspiracy, you need to come in thinking you trust no one and refuse to blindly take anyone's word no matter how close they are to you emotionally.
Start there and 'pure logic' will begin to open up to you. It is a fascinating journey with a lot of pain at the beginning but elation and wisdom at the end of the tunnel.
You need to not believe anything at all, whether the majority believe it or the minority do. You simply analyse, hedge your bets on truths and see over time which theories seems to contradict 0 of the others you have concluded in a scheme of pieced together truth.
If any single thing you believe in even indirectly makes another make less sense, you have either got to patch the missing link or accept that at least one of them is pseudoscience.