There is certain chemistry behind the process of crush. You can know it by understanding the process. You feel some unexplicable pleasant excitement in heart. Your skin on some parts draws more blood and as a result, it turns to more rosy hue irrespective of your skin complexion.
All these will take place irrespective of your marital status. Even if you do not have any real or meaningful work with a particular person, you feel like going to that person just to feel the presence. You can analyse whether you have done any unwanted transactions or purchases.
Another clear indicator is that you try to look at your best in front of that person. What ever you are doing every small cue or trigger reminds you that person. For example, you think of blue colour, you will get the mental image of that person when adored a blue dress.
You sit quietly and analyse, you will notice you will get lost in the memories of that person many times without knowing when you did.
