[49531]

There are tools you can use that will play a note like C and then another note, and you listen and guess what the 2nd note is. It just takes a bit of consistent trial and error learning, after a bit you get the hang of it and kind of create a mapping of what notes sound like in your head. I use a app called Tenuto for it, but I am sure there are others out there.

[sparky_z]

That would be practising "relative pitch", which is a standard part of ear training. "Perfect pitch" means being able to identify the 2nd note when played by itself, without hearing the 1st note to reference from.

[pclmulqdq]

I had an ear training teacher who claimed to have acquired perfect pitch by listening to a tuning fork for 10 minutes a day, and training himself to remember the pitch as a reference. I doubt very much that this worked long-term.