How do you know? Well, you have to be compatible, this is what we ask people to try to examine, whether you want to stay with a person to try to be compatible with that person, to have similar interests with that person, to be able to have a deep friendship with that person, to be able to communicate easily with that person, and most importantly to not want to change that person, to actually feel that you like them as they are, and you're attracted to their good qualities as they are.
Because if you ever go into a relationship wanting to change a person, then it's probably not going to work. Luckily for Steve and I, we liked each other as soon as we met, we were attracted to each other's good qualities, we hardly ever argued. Perhaps it was kammic. So it's a matter of really appreciating the other person as they are, and having a bit of unselfish emotion. And as everything changes, we need the willingness to change, and not hold on to making them the same. Because as you go through life, everything changes, and sometimes fear arises in the person, that if the other person changes and grows, they're not going to need you anymore. But that's ok, actually if you really have a genuine lovingkindness wish for that person, that's what you want for them, so it actually depends on the ability to have unconditional love for that person and joy with their good qualities increasing.
As well, it's helpful to have less selfishness and it also helps to have wisdom to understand that they might die tomorrow so you don't actually depend on them totally for your happiness, because nothing's permanent in this world. Then you don't overlook your own development, and depend on someone else's love in order to feel a healthy self-worth. This probably doesn't answer the question exactly, but I don't know, everyone has to judge for themselves who they're going to stay with.