Was just about to practice Death Letter when I see that Jack tunes to Open G, and it got me wondering, how does one know which tuning to go to? I mean, if you're writing something, how do you know to change the tuning-- and to which-- to get the sound you desire? I'm still a relative theory noob and this just caught me.

Takamine EG341SC
Godin Freeway Classic -> Boss TU-2 -> Fulltone OCD -> MXR M173 -> Boss RC-20XL -> Traynor YCV50 Blue
the tunings themselves only do so much to get a certain sound. it's all about how you play them. thats where music theory comes in.
I use alternate tunings to find easier ways to play what I write in standard, and to get a more open sound. I wrote a riff in Dm tuned to Standard, then tuned to open Dm and tweaked the same riff. Made it much easier. That's just me though.
For me writing in alternate tunings (is admittedly rare) but works in 2 ways

1) i'm already tuned to said alternate tuning for some other reason, and i start writing a song and then either can't or don't put it back into standard because it allows me to do certain things i couldn't before.
2) i'm in standard writing a song and hear in my head how i want it to go, then realise that's impossible without growing an extra finger, so see if i can transpose i to an alternate tuning.

1) is more common than 2) for me at least. That all said, if i hear a song here is almost no way i can tell it's in an alternate tuning until i try and play it and realise "that's ****in impossible" so it actually makes virtually no difference to the sound, it may just highlight things that you wouldn't normally think of in standard. (like putting in more natural harmonics etc.)
The only 6 words that can make you a better guitarist:

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