How do you know if the person is someone you want to be with for the rest of your life?
I think the easy answer is: you know. You just know.
I think the more complicated answer is this.
They make your life easier. Happier. They support you. They help you grow. They empower you. They never make you doubt whether you are worthy and deserving of love. They listen to you. They try. They make you a better person. They respect you. They complement you.
And, most importantly, you do all of the above for them too.