How do you know you're in the right relationship, or how do you realise you're in the wrong one?

Not sure to understand the question? How could you not know?

If you're thinking about finding the 'perfect' partner, forget about that. It's a lie. It's something fabricated to sell ads, page views and subscriptions to shitty services. Nobody (including you and me), nobody is perfect. So, no relationship can be perfect.

My spouse and I have been together for 25+ years and counting. It was quite obvious very early on we're fitting together nicely but that was far from being a perfect match and we had a few rough times and, all those years later, we can still have. But we've quickly realized how lucky we were to have found each other, and we valued that enough to get over any difficulty or hardship. And we still do.

So, to me that's a 'right relationship', just not 'the' right one as we could as easily have both been as happy with someone else if things had been different ;)

I second you on the point about a "perfect partner". However I still think a relationship can be "perfect", because a relationship is much more than just the two people involved.
As long as we accept that no realistic relationship is completely without conflict, I would say that a "perfect relationship" involves being able to work through the issues that inevitably show up. It's something you build together, not something you just have.
My impression is that the current "dating economy" breaks this perception a bit, to the point where a lot of people end up looking for the "perfect partner" that they automatically form the "perfect relationship" with, and aren't willing to do much hard work to build that relationship. I don't think that kind of "perfect relationship" (without any work) exists.
- As long as we accept that no realistic relationship is completely without conflict, I would say that a “perfect relationship” involves being able to work through the issues that inevitably show up. It’s something you build together, not something you just have.
  Isn't it what I just wrote? Real question (my English is lacking so...)

The wrong relationship one is a tricky thing to answer

For me, it has always been when the happiness they bring during the good times isn't worth the pain of the bad

When it's right, it's impossible to pin down

I have the most amazing fiancée in the world.

I've been married for many years, I've had other serious relationships

This woman is everything to me. Colours are brighter, food tastes better, flowers, sunrises, mountain views and all the other beautiful things are more wonderful because of her

It's like I was seeing the world with a dimmer before she came into my life

That's why I know she is right for me

I also choose this guys fiancé
- She's something special. She is spectacularly gorgeous, she turns heads everywhere we go, but she's only beautiful to me because of what she carries within her. She is the kindest and most compassionate person I have known
  Her eyes are so bright that they put a summer's day to shame, and in her eyes I can see her inner beauty shining through
  She has had many guys ask her to marry them.
  Rich, powerful, successful guys
  She couldn't care less. She has the biggest heart, and she's true to what she believes in
  I'm the luckiest bastard on Earth
This woman is everything to me. Colours are brighter, food tastes better, flowers, sunrises, mountain views and all the other beautiful things are more wonderful because of her
That sounds nice but be careful. You're talking about another human being here, not just a source of endorphins.
Make sure you don't neglect your own inner light. You are worthy on your own.
- I am worthy
  I've been on the receiving end of control and abuse. I learnt a lot from those experiences
  She makes me feel worthy, she makes sure that I know it

Red Flags and Green Flags.

Really, it's all about compatability.

One of the big red flags is how your partner treats "the help". Do they acknowledge the receptionists? The wait staff? Do they tip well?

If they're a complete asshole, and you aren't, that's a sign it's never going to work.

OTOH - if you're BOTH assholes, you'll probably get along fine, everyone else is spitting in your food, but together you'll be fine.

I think there are many people in the world a given person is compatible with. However, those relationships will be right in different ways.

A relationship is wrong if you can't communicate openly or you feel like they are trampling over your personality.

Confucius say:

"Relationships are like fart. If you have to force it, it's probably shit"

Do you trust them? Can you talk about anything with them? Is the sex good? Can you be honest to eachother? Are you happy?

If all are yes, you are in the right relationship.

If you feel alone in your head, ask yourself whether the issue is that you don't feel heard by your partner, or if you don't share enough with your partner

Wrong is a bit hard for me, maybe because "every happy couple is the same, while every unhappy one is unhappy in their own unique way" (para-quote from Tolstoy I think).

To me, I know what I have is right because I honestly feel like every joy we share is doubled, while every problem we share is halved. It's a cliche, but I honestly mean it. When I'm happy with her, it makes me twice as happy just to see that she is happy. I honestly feel like the greatest joy in the world to me is to see her be happy. Likewise, if something is wrong, I want to help her in a way I've never experienced with anyone else, and can feel in my whole body that we're in this together.

This doesn't mean that we never get mad at each other, but when we do, we've always ended up remembering that, at the end of the day, we're the most important thing in the world to each other. Whatever issues we've had, we're in it together to solve them.

To round off with Tolstoy: I guess "wrong" would be if any of the above didn't apply for whatever reason, that reason being unique in every case.

I'll keep it as simple as possible: How do you feel? Not minute to minute, but, overall. Like that quote from Maya Angelou that says, "people forget what you said, people forget what you did, but they always remember how you made them feel". If looking back on a relationship you realize you have too much heartache, that's the wrong relationship.

The people around you likely have a better understanding of it than you do. I’d argue that, deep down, you do know - but you don’t want to believe it if it feels like you’re not. Then a year after the breakup, you realize that you did, in fact, know all along.

Also keep in mind that "the right person" doesn't exist. Everyone has their flaws. It's a matter of what you're willing to deal with.

Are you happy?

Celebrating your 20th anniversary is a good indicator of the right relationship.