How do you go about practicing to play a note and immediately hear what it is? do re mi fa

First you need to familiarize yourself with the standard tone: A (i.e., la)

Then you can use the tone you hear and its intervals to figure out how many degrees it is in relation to each other, and then you can use your knowledge of music theory to figure it out

That's the way everyone does it, and that's the way my teacher teaches me, so you can practice it a little bit, and if you do it more often, it will be faster

What is it?