Dirk, the chorus and bridge usually go to a chord other than the 1 so don’t rely on those. For me the easiest way is the verse because they mostly start on the 1 chord which is the home chord and the tonic note will tell you the key. I should probably do a video on this to show you but start simple with a 12 bar blues. with a 12 bar you know there will only be 3 chords, the 1 4 and 5. and you know the 1 changes to the 4 on the 5th bar. then after 2 bars back to the 1, then on bar 9 it goes to the 5 chord and on bar 11 back to the 1. practice listening to these changes. once you can hear them go to a pop song which isn’t a 12 bar but still has simple and few changes. this takes practice like anything else but we can train ourselves to do it.