Finding what key a song is in is easy. It is 99 percent of the time, the last chord played in the song. If it's an Emaj, the song will be in Emaj. The song needs to feel completed so it will return the tonic(root) at the end to achieve this. Also, most of the time it's the first one too, but not always. It's conventionally the Root or the Dominant Chord that starts a piece. Beethoven's 5th Symphony is a perfect example of starting with the Dominant chord. It's in the key of Cmin, but it starts with G. In the end, the last chord is Cmin, so the key is Cmin.

As for riffs within the song itself, or as I would call "scenes". The chord progession will go through any of the chords (starting and ending), even keys. The scene will usually end with the tonic also, and the next scene will start with a new chord. One scene could be iii-vi-ii-V7 (repeat 3 times) then the fourth time iii-vi-ii-I. Then go into another scene (chorus) with the chords V7-ii-IV-I. It's not a rule to go this way, but it sounds good, and keeps the music moving.