Scales on a chord progression

Quite a bit of theory involved but;
To work out what key a piece is in, you would look examine the chords being used. For example, if your progression was: C | F | G | Am – I would recognise the scale needed as C Ionian – or C major pentatonic. A minor pentatonic would work also. You are looking for a key where you could find all these chords from. If anyone could correct me on this but I believe D minor pentatonic would work also.
For some heavy theory;
If you had for example: Dm | G7 | Dbm | Gb7 | Cb |, you have two groups of notes coming from different parent scales. The Dm | G7 comes from C major and the Dbm | Gb7 | Cb comes from Cb major.
Now if we remember our modes, in C major we have;
C Ionian (major
D Dorian
E Phyrgian
F Lydian
G Mixolydian
A Aeolian
B Locrian
So looking at our Dm – G7 I would use D dorian (or minor pentatonic), and we would use Db dorian (or Db minor pentatonic) for Dbm – Gb7 – Cb.
To be fair, alot of backing tracks let you know what scale you can use over the progression being played. But in a nutshell, its working out where the chords come from and how they link together. Sometimes there might be more than one way.

I’d like to echo what Ben said and then cap on it a little bit more. Make sure you play along with backing tracks that are simple and easy to follow. Some of them have really hardcore chords like augmented, suspended, diminished chords and they can be difficult for the ear to hear whether or not something really does sound right. I suggest starting with pop rock backing tracks or basic metal riffing backing tracks. Then take Ben’s advice and play each scale over the entire song over and over again. Your ear will start to become trained on which scales sound the best over which chords. And yes, Ben, I think you’re right about a Dminor working because dorian is a minor scale and you would just be using five notes of that scale instead of 7. I think that’s right. Unless someone else can come along and correct both of us, lol!