how can we tell when someone stops being a beginner?

Roughly speaking, an intermediate rating tends to start somewhere between 1200/1400 and ends around 2000.
This might seem like quite a 'high' rating to be classed as an intermediate. I believe it more so comes from the fact that most learning materials aimed at 'intermediates' are best suited for players from this rating range and higher, as players U1200 would still benefit more from tactical exercises and getting a handle on undefended pieces.
There's no point learning hours of strategical middlegame concepts aimed at giving you a slight improvement, if you drop a piece (or even a pawn) just through lack of board vision.
Somewhere around 1200-1400 is when most players start to phase this out of their games enough that one-move preventable material losses are not a reoccurring theme in their games.

Outside the 'chess world' though, I'd say a 1000 is where the intermediate starts. A 1000 definitely has enough comprehension of the game to the extent that they could play continuously against casual opposition and win at least half of their games.
I think chess.com places 850 at the 50% mark in terms of playing strength against the mass public.

From my view, that's complete rubbish. Everyone who isn't a GM is treated like a 'beginner' here.

Really appreciate your response Kraig. I came to ask the same question basically. You pretty much answered it, but I’ll ask additionally:

Instead of just the 50% mark for beating non-chess people, when would it feel like it’d be 90% or 99%?
For instance, KNOWING your opponent doesn’t play (maybe they’re a college friend or sibling), are easily as smart or smarter than you, but you still feel confident that you’d win even if you mess around a bit?

Is every game gonna be super intense like my life is on the line? I eventually want to relax a little. (I know “never underestimate your opponent,” but is that sustains? The pros seem to be able to able to enjoy themselves. Streams of Carlson are mind blowing. Sometimes he’s just like “oh, huge mistake, oh well” *recovers as if by magic*.

 Note: I have two accounts, this is my practice account. My main account daily rating hovers around 950-1050.

I've just started recognising my faults, the beginning of the end of the beginning I hope 😃 focused puzzle training is on order now. Turns out I really don't see sacraficial exchanges so that's what I'm working on, struggling with lower rated puzzles.

#1
"how can we tell when someone stops being a beginner?"
++ When that someone no longer hangs pieces and pawns

Only 3 divisions (beginner, intermediate, advanced) makes it hard. Chess has a very wide continuum of skill.

I agree getting rid of 1 move blunders makes someone not a beginner... but 1200 as intermediate already? So then what is 1600, advanced? If so then what is 2600, what is over 3000?

Among adult hobbyists 1500-1600 OTB is about average, so that would be at least 1600-1700 online. Among more serious players (who started young and worked hard) maybe a low master title is about average.

But "intermediate" aside, I agree that "no longer making simple blunders" is when someone isn't a beginner anymore. What we should label this not-a-beginner phase, I don't know.

You are brutally underselling the master titles. Although your expression "worked hard" is vague enough to defend what you wrote, I still find it ridiculous. It is safe to say that less than 10% of those players become masters, probably as low as 1%.

Fun fact, during one stream Jan Gustafsson and Peter Svidler had as a guest a 16 or 17 year old Alireza Firouzja.

They were asking him what kind of goals he had when he was a kid... and they had to apologize to the viewers that the grandmaster title was a good goal "FOR AMATEURS" (their wording, not mine) but for players like Firouzja (and Svidler) the GM title was always inevitable, and that's why talented young players never have GM as a big goal.

So I would say, if anything, I'm overestimating puny low titles like FM... but if you start young and are in the right environment, then an FM title is probably a reasonable goal.

Case in point, Levy had GM as a goal not too long ago, and he's just some amateur POS streamer who hadn't played serious chess in years... sure he gave up, but he's making a lot of money from streaming so it's hard to blame him. The point is someone like him had GM as a goal.

Now, for people who started as adults, like me (and maybe you) sure, it's very hard to get a title, and so NM or CM would be a very respectable goal.

And for adults who treat chess as a casual hobby, 1800 is above average and a respectable goal.

My main motivation for signing up to chess.com in 2019 was that the company that I had recently joined, had a a few chess boards in the diner, and a few staff dabbled in chess during lunch breaks. I was invited to play, and I still vividly remember my first move 1.H4, and didn't do too well. I joined this site to practice and haven't looked back since!

Once I reached around 900, I became competitive with the casuals - probably winning 50% of games.
Around 1300, I'd estimate I was winning 90% of games.
Around 1500, that's probably your threshold for '99%' confidence comes in (against casual players who have played less than 500 games).

There's an Elo Win Probability calculator which will help you with what you're after.
Just google that phrase. You plug in two Elo ratings and it'll calculate the win probability for the higher rated player.
eg. 1400 vs 1200, it calculates a 75% expected win. This rises to 92% if the opponent is 1000.

It is hard to tell whether you are trolling or joking or what. Firouzja and Svidler are very very very rare individuals, with extreme determination, resources and talent. They are the top 0.001% of players who are serious about chess as kids.

You disliked that I said FM is a goal, but I gave you a rare example to show that sometimes even GM isn't good enough for a goal. The point I've been making the whole time is that it's relative. This isn't trolling, but maybe some people aren't intelligent enough to understand.

So what happened is this: you stated that FM is "average" for a serious kid, I said that it isn't, and then you completely switched track and started talking about Firouzja and Svidler. What does that have to do with average serious kids? Nothing, they are a rare rare rare exception. The overwhelming majority of serious kids stop pursuing chess seriously and they get nowhere near to FM level. Your Firouzja example is totally unrelated to anything that is discussed here.

I think the real step is when someone starts calculating 2-3 moves ahead while keeping in mind a few candidate moves. Beginners all too often just see a move that makes sense to them and then play it. Key is not to play it but calculate different lines first.

Advanced levels (for amateur players) are reached when you start playing the game in your mind rather than on the board - the board becomes more a visual aid. A master can do it blindfolded, a GM 10 or more blindfold simuls.

Yea, I watched Magnus do a blindfolded simul against 3 opponents, on time (9 minutes). Insane.

@zone_chess, do you think that transition between playing onboard and playing in your mind comes naturally, or is it deliberate practice? That is, would you recommend, despite the discomfort, an amateur try to play in his or her mind, like the pros, instead of playing onboard? Similar to instructing a pianist to start learning sight-reading early as opposed to later. 

the average tournament chess player. there are a lot more casuals who don't know what the uscf is but enjoy a friendly game of chess time to time.