You will know if you found the right one when you cannot picture your life without him/her. When you also go through a lot together and find that there are no obstacles to kill the flame of love, then you will know that you have found the one you will spend your life together for a long time.
I don't know if there really is an answer to that question. I think finding the right person is pure luck. Some find it First time & others spend their life trying to find the right partner that enhances their life only to find their life only becomes more difficult with the wrong partner. I say be happy & surround yourself with great friends. That is what truly matters. Count on no one to be responsible for your happiness. Allow no one to control your life. That is your responsibility & you will never let yourself down or disappoint your hopes & expectations. You also need a good dog in your life. They love you no matter what & are what loyalty is all about. They willingly segregate themselves from their own kind to spend their life with you.
You know you have found the right person when you are willing to do anything for them but they don't ask for anything. When they will do anything for you but you don't ask for anything. When you each have a flitter in your soul that only the other can qualm. When you are sick and he is willing to hold your hair back. When he wants to watch the game and you call his best buddy to come watch it with him. When you get on each others last nerve and think this is something we will laugh with our kids/friends about.
You can't. Besides, why are you asking this question? Do you have strong feelings for more than one person? Then you're indecisive. If not, then you have a fear of commitment. You can't *know* if there even *is* a "right one". Can you unconditionally commit to making this other person's life better? If not, then it's not true love. That doesn't mean you throw it away, but you recognize it for what it is.
You can't know. You can't read the other person's mind. The other person could be a much better or worse person than they appear to be. Unfortunately we have to rely on gut feeling and past behavior to judge people. You can feel like you know, but you can never really know, even if they show themselves to be a perfect match for 80 years. Really all that matters is that you both meet each others needs and don't cause each other to much emotional trauma.
If you are asking yourself "is he/she the one?" its quite likely they are not. Being with someone for the rest of your life is a heavy thing...it's an important thing. Life long partnership takes commitment, selflessness, determination, perseverance and forgiveness. It is a journey, not for the faint of heart. I encourage you to take time to learn about yourself. To be honest with yourself about your strengths and weaknesses and to love and accept yourself completely. Self love, confidence, and inner peace will open doors to opportunity you never knew existed. That's where you will find "the one"