How do I know if girls are intimidated when I look at them or they are not interested?

When I'm with my guy friends sometimes they tell me , that girl is repeatedly looking at you but when i turn around to see her , they act like i don't even exist 😂
And in public places, when i look at girls, they seem intimidated/not looking at me...
How do I know if girls are intimidated when I look at them or they are not interested?
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