When determing a key from a chord progression these rules apply (in diatonic keys):

1. two minor chords a full step apart are ii & iii chords

2. two major chords a full step apart are IV & V chords

3. a minor chord and a major chord a half step apart are iii & IV chords

4. a minor chord and a diminished chord a full step apart are vi & vii° chords

With these rules a root chord of a MAJOR key can be determined. There can be out of a key chords in progression, but the rules still apply. If the song is in minor, then relative minor is a root.