When tabbing a song, how do you figure out what tuning the guitar is in?
Try to listen for individual open notes, the low 6th string is the best place to start, if you can recognize it's an open low 6th string being played, try tuning it to the same tone as what you hear
Personally, I would start with the low string, listening for the lowest note. As you tab out the rest of the song, tunings may become more obvious as some notes are more likely open strings or fingering lends itself to a certain tuning. It's also worth looking at other tabs of a band's work to see what tunings they commonly use.
Guitars & Gear:
Parker Nitefly M
Sumer Metal Driver
Ibanez RGD2120Z
Two Notes Torpedo CAB
These answers are totally correct for the most part BUT on occasion a band will throw a curve ball. Take AC/DC for example, Angus and Malcom are not very complicated musicians at all. Blues based Rock and Roll std 440 tuning, that is until you try Night Prowler. It can be played as std 440 and some work meaning it is almost complicated to play but if you tune to Eb, the whole song falls into back to what feels like basic Angus and Malcom.
Yeah what has been said
Depending on style, it can be tricky to hear open strings. Try to find the lowest note/chord that is being played and tune to that as generally it will almost always be used - you can check other songs from the same album and if you don't find lower then you've almost definitely got it.

If you hear stuff like hammered on power chords or otherwise very fast changes that would be tricky in a regular string structure, it's most likely a dropped tuning