How do you know if she/he is the "right" person that God sent to you?
Good question.
I think the answer might be found in the question. When we say "right person for us" what do we mean? Some people might think that to mean the person who will make us happy, fulfill, or complete us in some way, but I think that's the wrong way to look at marriage.
When I was a kid (15-16) my dad got me a car. I was looking for a car. I wanted a car and I got one! My dad got me a car. It was old, ugly, and didn't run very well. It cost me money. It left me stranded sometimes, but it was my car and I loved it.
No car is perfect. They all break down from time to time. They all cost money. I could have complained. I could have said; "dad, this car is not for me. It's not very cool. It breaks down and causes me time and money". But, I never did. I was happy to just have a car.
I realize we aren't talking about a car here, but I can't help thinking the sentiment is the same. Fact is, people (spouses) will let us down. No one is perfect, and if we are looking for that perfect car then we are in for a great disappointment. all cars require time and effort.
All relationships are very much like owning a car. We must be willing to put in the time and effort. So, I am going to turn your whole question around. You asked,
How do you know if she/he is the "right" person that God sent to you?
When you should be asking; "How do you know if you are "right" for she/he that God has sent to you?"
Can you put in the time and effort? Do you know how to do all the maintenance this car will require? Are you willing to learn and do what is necessary? You might find yourself stranded a time or two.
For cars I'd advise membership in "Triple A" which stands for American Automobile Association, but for people the only association you need is God.
Are you right for someone? Yes, and that someone is right for you. You will know them best when you know yourself best.