Finding a perfect life partner is not an easy task. To be together for a lifetime, your thoughts should match with the partner.
One has to understand each other's likes and dislikes and see if you can be with each other for life.
On meeting a stranger for the first time, if you are feeling safe and comfortable with them, then understand that you have liked their company.
To understand the body language, see whether the future partner talks to you with eye contact or not, whether he is interested in listening to you or not.
How much he wants to know about both his personal and professional life will tell you how serious he is about his future with you and how interested he is in your life.
Two unknown people meet on a date to know each other. What kind of questions they ask on the very first date, their personality can be understood from this.