The real answer to that is, you don't. Unless you've been heavily influenced by a number of romantic comedy movies, "the right one" is a myth. The truth is you aren't destined to find the "perfect one", because if you do, you'll find yourself searching for the rest of your life with high expectations.
Now I'm not saying to settle for less than what you deserve. I'm pointing out that relationships not only involve that 'spark' or 'chemistry,' but it also involves that effort factor as well to ensure that you are happy with the person standing right next to you.
To find out whether or not you and your partner are compatible for a long-term relationship, here are some simple keys to follow:
Communication: unless you're both psychics, no one can read each other's minds. The best way is to always communicate exactly how you feel, and never internalise your emotions and thoughts. Because by hiding yourself, you are not being honest to your partner and the relationship, which is a significant part of analysing whether or not you should continue to stay with your lover.
Honesty/Trust: as pre-mentioned, you should be honest and trust your partner with everything. If you're being yourself and your lover accepts the 'true you,' then it's obvious that you've found someone who you could potentially spend the rest of your life with.
Respect: as well as love, you should always be caring and considerate towards your partner in a way that respects his or her values. If your partner has similar values with you, appreciate them. If your partner has different opinions from you, respect them.
Commitment: the last key element to realising that you've found the "right person" is of course commitment. Does the idea of staying with this person for the rest of your life excite you or scare you? Could you accept all of their flaws? Are you willing to commit yourself and put 110% into the relationship? If so, then you can expect a long-lasting connection.
It's important to note that the true element of the entire experience of a relationship should involve happiness. Even through tough times, you should always have the strength and happiness to stay with your partner.