How do you know if someone is just infatuated with you or really into you?

Infatuation is usually what happens in the beginning of every relationship when you meet someone new , they compliment you , they speak highly over you , they make you their top priority etc.. but eventually that infatuation can disappear or continue , When do you know it’s actually legit?
How do you know if someone is just infatuated with you or really into you?
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