How do I differentiate between modes?
Suppose a person is playing "D Dorian". How do I know that it's "D Dorian" and not "C Major/Ionian" or "E Phrygian" and so on, and in fact "A natural minor" because it has same notes as C major.
When you play a C Major scale and have emphasis on a note other than C, such as D, then it is D Dorian. The player implies that the D note is the root using the notes of the C Major scale. Another thing is the harmony behind the scale. What chords are being used? That would direct your ear in hearing where the root is. If I play Dmin7 chord and use the C Major scale, I will land on the D note for a completed phrase and that is the sound of Dorian. Each mode has a particular sound. Play each note in the C scale to its first octave ( D to D ) and you will hear the flavor of the mode. Do that with all notes in C Major Scale.
Yes, they're using all the same notes, but not necessarily in the right order...
C maj. will be CENTRED around C, D Dorian will be CENTRED around D, E Phrygian will be... you get the picture.
The home (CENTRED) note will be the mode letter. The chords may well be the same, but their function will be different, i.e. in C maj., the G will be the dominant pushing back to C, but in, D Dorian, the same G will be the sub-dominant, a chord to visit, but not say "you're going back to C".
If you look at just a small part of the tune under a magnifying glass, so to speak, then you don't know. A D minor chord is playing, or a G7, and all the notes are from the key of C major. You could be in a II-V-I cadence or whatever.
To know that you are in D Dorian, you have to step back and take a bird's eye view of the tune's structure.
Strong clues about the intended tonality will emerge. For instance, if major sections of the tune keep starting on a D, with a Dm or Dm7 chord, and return to Dm, that suggests that the tonality is D, and so if the notes selected are from C major, then the tune is being composed in D Dorian.
Another clue is if that it is a tune from the jazz and blues tradition, the bluesy alterations heard in the music, such as the addition of a tritone, will be with respect to D.
The music might contain V-I cadences targetting Dm, such as A7 Dm, suggesting the key of D minor related to F major; however, these may show themselves to just be a temporary tension heightening device which confirms the D tonality.
For instance, consider a vamp like Dm G7 | Dm G7 | Dm G7 | A7#9 - | Dm. That would be clearly interpreted as Dorian because of the Dm/G7 interplay. The A7#9 is just a bluesy turnaround.
The basic idea is that we would like the effect of a dominant to target the root note D in a powerful way, and so we just use this A7, without caring that it uses notes from outside of the D Dorian mode.
Note that an A7 dominant is "foreign" to the D minor tonality itself! To obtain the dominant, we have to alter the natural minor scale to have a leading tone. The notes of A7 are found in the harmonic and melodic minor variants of D minor. The same way that D natural minor can get a C# leading tone to support A7, we can temporarily introduce a C# in D Dorian music for the same reason. So the point is that there can be some "distractors" that are off scale, yet the music can be in Dorian.
A "mode" is simply a scale that has been altered in some way. The unaltered natural major scale is playing in Ionian mode. The unaltered natural minor scale is playing in Aeolian mode. The other modes can be constructed by altering the natural minor or major scales.
For example the Phrygian mode can be formed by a natural minor scale with a flattened 2nd.
A good explanation of this is given here: http://www.mandolincafe.com/niles2.html