How do you determine diction in a song?
This question could be taken many ways. I am assuming that you mean how does a song-writer determine their word choices as they write music. This is the most logical avenue by which to answer your question, so I will proceed from that frame of reference.
A song has beat, meter and rhythm. It may also require rhyme. This sounds a lot like poetry. So, in choosing words a song-writer might think about many words that rhyme that could work into the end of their lines like the old U2 song: "deviate, hallucinate, try not to hate." Another consideration they think about is the amount of beats or syllables they can use to convey a line or message. For example, they may want to use the word 'change', but need to use 3 syllables or beats. Then they choose a work like 'modify' instead.