How does a 12 year old girl know when a 12 year old boy likes her?

It is not necessary for you to know if he is going through Puberty as all boys will go through it just the same as girls will. If he hasn't started by the age of 16 then seek a Doctors advice. However. in my opinion, you may get some insight by looking back at your behavior duringf those years. Seems to me that this is when thy want to be anywhere but with Mom and Dad. And begin to think they have more wisdom as well