How do you determine which leak is accurate and which one isn't?

Again, lots of "credible" leakers leaked false information prior to the Switch launch.

One can wish or expect for Switch support to continue to happen because Sony and MS are doing it (for digital ecosystem reasons more than software sales), but it runs counter to Nintendo's history.

Did Mario 3D Land and Pikmin 3 release for the Wii? Pokémon X&Y or A Link Between Worlds for the DS? That was a 250m userbase...

In fact, one can argue the only major Nintendo games that deliberately release after a successor console is out are reused asset Pokémon versions (Crystal, Emerald, B&W2, US&UM).

What the source is and how it's being reported. If Jason Schreier is reporting on something for example it's almost surely legit. There was a bunch of nonsense about the Switch but the hybrid system detail was so widely reported that it was basically all but confirmed.

In a few months time, this is going to be a "Sorry guys, I need to adjust my previous sayings, plans have internally changed at Nintendo and they want to release the system in target of Holiday 2024 or Early 2025".

Casting the net so wide they simply can't be wrong

Riddler Khu, one of the most prominent and accurate Pokemon leakers, has now stated that the Pokemon details from the leaker in the OP are true at least, Terrestalize will be like Z moves and Gigantomax in that there is both a for all-Pokemon mechanic and a for special Pokemon only mechanic to it, as some Pokemon will get unique Terrastalize forms in the DLC:

He did however decide not to comment on the Switch 2 part of the leak:

So, the question now is, did this guy get real Pokemon information from somewhere and then decide to add made up Switch 2 rumors to it, or is he legitimately from a Pokemon outsourcing studio that is working on a next-gen Switch patch for Scarlet and Violet like he claims?

The won’t confirm the Switch 2 until the end of the year, similar to what they did with the current switch which was unveiled in October 2016 and released in March 2017.

Here is Jason "reporting" on the Nintendo Switch releasing in 2016 which didn't actually happen.

https://kotaku.com/possible-leak-that-nailed-pokemon-codename-also-lists-n-1761518611

So again. How do you know which leak is accurate and which one isn't?

To be fair, he is commenting on another person's leaks in that article, it's not a leak based on his own sources. Schreier has overall the best track record in the whole industry in terms of leaks, just as a for instance, I recall him accurately leaking details of every Assassin's Creed from 2011's Revelations through 2018's Odyssey, it got to the point where Ubisoft had pretty much blacklisted him, leading to him not talking about Valhalla at all until another leaker talked about it first. And those AC games were just some of his many accurate leaks. Now obviously nobody is 100% accurate, but in Schreier's case, his misses usually seem to be because of internal delays or cancellations after his initial leak, rather than bad sources on his part, though he did have bad sources a few times over the years. 

In this example you'd look towards the end of the article and see that he said he hadn't been able to confirm it with any of his own sources so it wasn't something he was vouching for. Shikamaru went into his high accuracy but the main point is what the source is and how it's being reported. If a console is launching in under a year and leaked details about it are being widely reported on then there's gonna be a lot of accuracy to it. As I said before anyone paying attention to leaks in 2016 knew that the Switch was gonna be a hybrid system ahead of time so leaks clearly have value.