In most cases, you know a girl likes you when she allows you to put your penis inside her, but sometimes the interaction gets cut short and you’re forced to get her number.
If a girl asks you to call her after you save her number in your phone, she really digs you.
If a girl doesn’t like you, she won’t care to get your number because she screens unknown numbers anyway. Having your number is a “just in case” move she only does on guys she wants to see again. Being aware of this lets you know which girls you can take a little more seriously.