There is no magic or mathematical formula for such things.
There are no such guarantees in life in this matter and many others.
You can use the information and evidence that you see. How he or she treats you and acts with you.
That is all anyone can do. Some people are fortunate, others less so.
The KEY to true Love and Romance are not like recipes that you can use for that perfect relationship. That Key is unique to every couple and relates to their social cultures and values. Almost every human relationship has its ups and downs. When you can focus on the important and valuable stuff that makes it all worthwhile, rather than curse the stuff that causes pain then you are on the way to finding that right person.