[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.

[Tor3]

That's simple pitch memory. I started playing guitar at 12 and at that time used a tuning fork - and I noticed that after a while I could hear the "right" note of the A string in my head and I didn't really need the fork anymore. That ability is just pitch memory (which can be thrown off) combined with learning the timbre of the instrument you play (in my case a guitar string).

And yes, that's not the same as perfect pitch, which typically includes much more than the above. I certainly don't have it, even if I can pick any string first time in the morning and hear if it's "off". Because that's all I can do. But then again I don't get annoyed by "out of tune" sounds in the world around me, unlike what people with perfect pitch sometimes tell about.

[C-x_C-f]

I don't have perfect pitch but if there's no sound around I can accurately recall the pitch of my alarm (the standard Android one) within a half tone, so I think that strategy makes sense