I’ve been thinking about this for a while cause I’ve been hurt a lot by people i felt actually were sincere to me

But then, what is 'sincere'?
Anyone being true to their real self?

Tough question! I can understand your feeling, and at times you may feel frustrated! Often wot happens is, we begin fantasizing about the other person's feelings, something that revolves around our expectation of a reciprocation. But relationships, let alone life, ain't arithmetic! Hence, I feel, being observant about ones own feeling and it's causes can go a long way in gauging the other person's true involvement in the relation.
Welcome to M2, and all the best

The fact you got hurt doesn't necessarily mean the other isn't sincere.
Thing is, you need to have very clear established, healthy boundaries and be aware when something feels off to you and then do something with that.
I think most people get hurt because they aren't paying attention to what's going on. Not really.
That has happened to me as well, and I think to most of us.

Patience is the Key to Everything. You cant get Chickens by smashing them You have to wait till hatching them .

The life is full of risk you get hurt by someone and another person put a smile in face

Myself I always trust my instincts. but it's so hard also .

Totally agree. Even two person are sincere, love doesn't guarantee one of the person won't be hurt. Believe what you want and try to work on it, good luck!

Online dating dating adds a level of distrust, not only do we need maintain vigilance in meeting people, but now we even have to be careful they are what they claim to begin with.

In my opinion, you cannot take people at 'face Value' on internet dating sites, that is exactly what scammers want you to do. You can get to know someone online by emailing/chatting before deciding on meeting in person, that may be the only way if they live very far away, but meet each other as quickly as possible if there is a mutual interest. You avoid getting hurt online by keeping it as friends only, don't become too invested in any other form of online relationship, until you date them in the real world you don't know if it could be any more than friendship, it could end up just being the one date. Good luck in your search.

Actions speak louder than words. That's all.

If you want to know if somebody is sincere, you need to give them the opportunity to show you. You can't just take their word. At the same time, you can't keep testing them. YOU have to make the right choices and take responsibility for your actions. Everything that has the potential to feel good also has the potential to hurt. The important thing is to be who you are in both situations.