When your heart speeds up at the thought of them or you can never stop thinking of them. When you get butterflies in your stomach when you see them. It may sound cheesy but thats how you know you really like someone.
Ask them.
You get to know the person!
The brain will know if you like someone or not. Your instances tells your brain.
warn the person you like
In my opinion, you start to like someone when you get to know them. Usually you like someone because of their personality. It comes naturally when you start liking someone. It can be when your young, or old, it doesn't matter. You'll know you like someone when you start being more nervous around them then usual. It helps if you know they like you too.
You can have a crush or be infatuated from afar with someone you don't know or even may know a little but when you feel you like someone you will like them as an individual and know them.
ask them. or get someone you know to ask them
If you want to be with them
an apprentice is like when someone is following someone to learn something. You know, like master and apprentice?
Love at first sight
Ask him, get to know him better!
talk to them get to know them better