I'm trying to learn how to transpose music to tabs by ear, and my question is how do you know which tuning a song is in?
By ear, do you mean totally just by listening to it or listening to it then playing it on guitar or what?
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That's pretty impossible I think...
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Quote by asfastasdark
That's pretty impossible I think...

well unless the song plays a lot of open strings
You can figure out pretty much any metalcore and deathcore by ear. Mainly cause most of it's based around similar principals. Same with Melodeath.
Yeah...for low tunings it's not really a problem, all you need to find out is the lowest note used. If it's lower than E it's most likely some Drop-D or Drop-whatever tuning, although most more traditional heavy-bands tune their guitars down a full step instead of using Drop-D.
Also, figuring out the key of the song first helps. If it's Eb minor or sth. like that it's most likely tuned down a half step

Open-Tunings or irregular/modal tunings however are often hard to do, you have to listen for open strings in that case, since the chords are often build around those.