Having went to university for music, I know better than to identify the time signature of someone else's music unless it's extremely well documented and agreed on.
On one hand 12/8 is pretty safe, because it's a standard blues form that has been around forever, but identifying someone's song as 12/8 is just asking for them to tell you it's actually 6/8. You might even misidentifying a song as being in 12/8 when not only is it 6/8, it's actually 3/4. It could even be common time with a triplet feel.
It's something of a tough one, because a lot of songs you would identify as being 12/8 can also be conceptualized different ways, and might have been conceptualized a different way by the person who wrote it.
Calling out a time signature or a song's key or a chord progression by the technically correct name by ear is one of those things where you better be shooting to kill, or else someone is going to come along and be like, "Well, actually....".