There is but one way to know if someone is "the one".
You first have to know yourself!
Know what traits (you're looking for in a mate) and then spend enough time with them to (confirm) they have those traits!
Most people allow "impulsive connections" and "happenstance" to dictate their relationship choices.
That's the equivalent of going shopping without a list!
Only (you) know what is on your "must have" list.
Never separate your mind from your heart when making relationship decisions. The purpose of the mind is to protect the heart.
Ideally the goal is to find someone who shares your same values, wants the same things for the relationship that you do, naturally agrees with you on how to obtain those things, and last but not least there is a mutual depth of love and desire for one another.
There is no amount of communication or work that can overcome being with someone who does not want what you want.
Like attracts like and opposites attract divorce attorneys!
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde