This is two questions since they are sorta similar

First how do you distinguished if a song is in its major vs relative minor, like C major vs A minor. I can understand in simpler cases where it starts on the 1, and ends on the 1, so the resolution is clear. But what about cases where the 1 chords isn't the first chord?

The second part is how do you do this in general? If a song doesn't start on its 1, how do you deduce the key? Do you just look at the specific notes used? Is it an ear training thing? and sorta loops back to the same problem as question 1. If you find it's usinf a certain scale, how can you differentiate between the major and minor?