(PS: feel free to ignore all my previous questions about transcription as I'm starting to become redundant.. unless it is too late to ignore) (PSS: thanks Bill <3)
the way you have to do it is you just have to know. the more songs you hear, the more this will happen. for example, a song like katy perry's "teenage dream" has no Bb chords in it. but i know it's in the key of Bb. i just know. the chord sequence in the song "ain't no stopping us now" always starts on Bbmin, but i know the song is in Fmin (or Ab). i just know. It's like when you see a face sideways. you know the face is sideways. how do you know? i don't know. you just know. that's one of the weird mysteries of the human mind