For instance, if I play the middle C or any note before a song starts, I can instantly guess the key of the song(I think this is known as RP - relative pitch).
Another trick I've used is finding out the lowest note I can sing with my voice and use it as a "reference pitch" to guess the key of a song. However, sometimes I have soar throat etc. so this is not a reliable way to find the key.
So the question here is if I have no instrument around, how can I figure out the key of a song? Can you memorise a certain note(like the middle C) and go from there or how does one train to achieve such a thing?