There isnt any specific way to tell but time. The more time the better. Unless your in a situation to where there is more than one person that you think might be interested in you, you find out which one has similar qualities as you and move from there. Now that time isnt of the essence, you should watch for touching signs: ex. A slight brush of the hand down yer arm or lower back. Or a extra long stare or maybe even a few glances at you should tell you that they are interested. If they are the "one". Qualties, personality, makes you laph, and of course...... GREAT SEX!