Modes

Greetings,  I'm new and learning.  I was viewing the information on different tunings for noter/drone playing.  Am I correct in thinking that if I'm in DAA and want to move to a different mode, I just move the tonic note for each mode to the same location as the tonic D in DAA?  I would take the tonic for aeolian, mixolydian, etc. and retune so the tonic "D" fret in DAA becomes the tonic fret for aeolian, mixolydian, etc?  Also, how do I know in which mode a song is written?  Thanks.