The answer to the question, ‘How do I know the right one?’ is straight forward.
You should have your requirements. And anybody who satisfies those requirements should be the one.
At this point you still have to be careful by being certain that you meet those requirements yourself. Why is this necessary?
Because you need to be it and feel it in order to know when someone else be it.
So ask yourself first, ‘Are you the right one for someone?’