Flaw! Humans are imperfect. If someone is perfect it just means he hides something thus dishonest. If someone flaw is visible then there is fair chance he is honest.
You can tell through their expression, tone of talking, your relationship with that person, words used by him. Usually friends, brothers and sisters are not sincere when talking. So don't take their words to heart.
That's the thing, a lot of times in my life I met people saying they wish for honest, sincere people and when I was being that with them... they either disappoint or betray. I knew a girl who would ocassionaly go crazy for no reason. She would go mad, rage and make no sense... even then, I would honestly listen to her even when she was going crazy. After years, she confessed because she felt guilty, she said that in the past there were a lot of people who were insincere with her and would betray her, so she went crazy and did all sorts of stupid stuff because she wanted to test me and my honesty. I already knew this, I am not all that smart but after years you would eventually figure it out... so, I knew she would intentionally act crazy to test my honesty and sincerity. I felt sorry for her, as she is doing the things that she hated the most when it was done to her. My ex after years in my relationship, she confessed that our meeting was not a "coincidence" but rather she was stalking me. She confessed because she felt guilty. What I am trying to say? I hope everyone would treat me with honesty and sincerity but just because I am hoping something does not mean I avert my eyes from reality or not aware of the fact that I am being taken advantage of. Yet, I do not want to end up like the majority who are desperate in searching for someone who is honest... rather than waste time search and worry about it, it is better to try to be what you wish others to be like. I mean, the amount of effort and time it takes to improve yourself would leave no space for having false expectations. Basically, be the best you and if you meet someone who is trying to be sincere like you, then you are blessed. If not, why waste time in a whole... keep moving forward and improve yourself and eventually happiness will be found. Edit: For the OP, if you yourself put a huge amount of effort to be honest with yourself and those around you, you will eventually be able to discern between honesty and lies. Also, if you yourself keep moving forward and strive to be the best you... you will find yourself eventually surrounded by honest people, without even trying you will find honest people to surround yourself with. There is no need to search or worry about that, what you need to worry most is you yourself.
I think for me someone sincere is that when something happens to you or not, they are there for you no matter what. For better or for worse. They want to be involve to you even for the little things.
People aren't easy to read as it seems, especially adults. I don't rely on face to face judgment, instead I look into their friends, families or anyone who can link up to them, well it has become my hobby to know what's the background of the person first to trust that person. I try to look up on them on social media accounts, but I don't trust them either since there's a lot of fake accounts. All in all, it's better to look for someone you already know you can trust, even I'm a social awkward, a lot of people who are good at socializing becomes my friend that I end up relying on them when it comes to information of people. That Said, it's better to improve your social skills to judge a person, either way I've seen a lot of sincere people, and most of them will just talk to you honestly. Well I'm a socially awkward person, so a lot of them easily shows their true colors around me, and these colors aren't the positive ones... so I don't have any problem if this person is detestable or not, though it also hurts when they are too blunt on not showing interest in me. I actually have a real life tsundere childhood friend, ... only to end up being ntr because of my awkward social skills and low esteem mindset, I regret it till to this day that I didn't answer her properly even when it's mutual, damn it. Knowing a tsundere is actually easy, the number one requirement is that you're close with that person, then anything else becomes easier to read, tsundere-esque characteristics will not happen on a stranger, it would be weird if that happened. Edited: You can do multiple reply in one message Also Read Manwha Skill of Lure and Love Riron, those mangas teaches awkward people to become better in socializing. Naze Toudouin Seiya 16-sai wa Kanojo ga Dekinai noka? is an example of "do not do this", it's a good manga by showing the mindset of awkward person.
oh what happened then? did you beat that person? how about your other friends? they know what happened?
I think it depends on how much attention they voluntarily spend on you (forced time where they don't have a choice doesn't count). If 90% of their free time is spend on you (even bullying you), then you can think about why they're so fixated.......