How to know if a girl has crush on you?

I have been told by my friends that I look smart and if they used to look like me they would have a bunch of girls around them... but I never found any girl crushing over me... although even my ex girlfriend used to tell me that girls do like u... but how come i never noticed a single girl who likes me... i am a shy guy who doesn't really interact with many people... is it the reason?
How to know if a girl has crush on you?
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