How do you tell if something is canon?
First, find the official source for that something and verify its full accuracy as you currently understand it.
Second, find any official source made later which contradicts an element of the original something. If you can’t, it’s canon.
At its simplest, that is all you’re looking for.
As with any field, though, there are nuances and loopholes and blurred voids between that few navigate. Unfortunately for us, these aren’t so much deep, exploitable lore secrets as they are confusing ideas that take far too much time to explain from scratch.
That all said, what you shouldn’t be afraid of is asking for help or explanations. If somebody claims something isn’t canon, kindly reach out to them or others for more information. Then check for yourself. If it appears to be correct, congrats on learning something new! If it seems to be a misunderstanding on their part, remember that nobody enjoys being wrong. Discuss it and let them know, but don’t be an asshole about it.
But what happens if everyone is wrong about something?
Fanon (fan canon) is a corruption or exaggeration of canon. It is usually a popular misunderstanding of a story element attributed to a particularly underdeveloped or poorly communicated piece of canon.
Due to the collaborative nature of fan wikis, fanon can spread quite easily from what was hopefully an error. If the community does not have enough members who are willing and able to check against these kinds of errors then the information can go unchallenged for quite some time.
This has happened many times before in Magic and will probably happen many times again. We’re human and some things are just confusing to read and record.
But the way you fight against fanon is the same exact way you find out if something is canon. With fanon that usually means getting entirely locked out at that first part though. Still, if you’re particularly doubtful of your own abilities after finding nothing but don’t want to make the grand proclamation of stamping “FANON” in red ink just yet, put out a call for friends and other fans to check for themselves and help you.
At the end of the day, be nice to people and work together to make your communities better. Also, it’s important for you to understand that you don’t have to do any of the stuff detailed here to be a real fan. Canon is one small corner of a fandom and you should do what you enjoy at your own speed.