Personally, I believe that love is not possible at first sight. Yes, you can judge a person by how they walk, talk, dress etcetera but that also takes more time. What you see at the 'first sight' is only the first impression. You can't know a person by how they move, talk and look. Every human being is diverse and unique. We all have idiosyncricies and things we try to hide. I believe that if you listen to the Holy Spirit and are deeply imbedded in God's Word that he can show you your mate, but it takes time to know them. You can't rush a relationship if it is to be properly nurtured and to be healthy over the years.
Love at first sight is not love at all but infatuation. You need to be around that person, you don't feel complete without them, the emotion comes too quickly, and jealousy and distrust are very common factors. Those are some ways, just a few, to recognize a infatuation. Love is comprised of two people coming together as whole individuals, they don't need each other. They just want to be together. They truly enjoy being with the other person but don't feel like they are falling apart without them. Trust is rarely compromised, and if it is, it is easily forgiven. Love is quite different from lust or infatuation. And in my opinion, any relationship that begins with infatuation ends up with someone getting bored or used.
Relationships are built gradually. I learned that if you really want x person so badly see why they appreciate doing and help them out with that passion.
That way you have an even better relationship.