Question from my comments a few posts back: How would you recommend a person go about discerning marriage to a specific person? How does one know that the person they're with is THE one?
First of all, let me be a wee bit of a jerk and speak to a grammar issue which has been bugging me for years. Despite how politically incorrect it has become to use the masculine pronoun to refer neutrally to both sexes, it is still important to observe the rule that pronoun numbers must agree. "They're" is plural and should not be used as a substitute for "he or she is." A better wording would be How do I know that the person I'm with is THE one? See this site for more information on grammar usage.
Now to answer the question, as best as I'm able.
In the true understanding of marriage, the moment at which you know the person you're marrying is THE one is at the exchanging of vows. The vows alone make the marriage happen, because when you make a solemn vow you make it before God himself, and he holds you to it. We are not to expect some sort of cosmic awareness of destiny when we look to marry. In any dating relationship (more accurately called a courtship, since the purpose should be to discern marriage to that person) you will always have a degree of uncertainty about the relationship right up until the moment the vows are exchanged. As I prepared to marry my wife I was plagued with doubt (which varied from day to day) about the decision right up until the moment we made our vows.
So if you find yourself in a relationship and wondering about its future, ask yourself if you are willing to make those solemn vows with that person. Are you willing to commit? Are you willing to tolerate that person's faults and annoying habits? Is that person equally willing to tolerate yours?
Think back to the characteristics which attracted you to this person. Are those positive attributes still present? Are you taking those characteristics for granted because of the now obvious faults? Those things which annoy you about this person will not go away; marriage will magnify them. As the German satirist Georg C. Lichtenberg said, "Love is blind, but marriage restores its sight."
It might be helpful to make a list of the things you like and the things that drive you batty about this person. [I would not recommend sharing that list with him or her.] Carry a pen and paper for the next few days and add items as you think of them. Give yourself about a week to complete your compilation. Once your list is ready, assign a value to each item which represents its importance, between +5 and -5. For example, "Clips his fingernails at the table" might represent a -1 for you, whereas "Holds the door open for me" could represent a +2. Then there are the big ticket items, such as "Doesn't have the same faith beliefs as me" which is an easy -5 and "Gets along well with my family" to which I'd assign a +4. Add up the items in each column and use the final sum of the "good" vs "bad" values to help you in your decision.
A word of caution with this method: please do not use math as the sole method of discerning marriage. It also helps to talk to people you respect and trust and get their input on your relationship. Share your deepest concerns and be open to any insight they might have. Share the items on your list with a couple who has been married at least fifteen years and ask them to rate the importance of each item to make sure you're not off-base with your own ratings.
I also cannot over-emphasize the importance of prayer in discerning marriage to a specific person. God is intimately concerned with who you marry, and it could even be said that he knows who it is to be. But that doesn't mean that once he leads you to that person that everything will click and you'll fall in love over the course of an evening like Cinderella and Prince Charming. It is never that simple. You must always follow his path for you, especially through the courtship itself.
Ultimately, it all comes down to your decision, and to the level of commitment you are willing to invest in the marriage. The word decide has its roots in the Latin decidere which means "to cut off." It contains an element of permanency, and so should your decision.
I pray that you make it well.