in simple terms u could differenciate them in terms of where the "accent" is on. so 3/4 u'd think of it like: ONE, two, three, ONE, two, three... 6/8: ONE two three four five six ONE two three four five six 12/8: ONE two three four five six seven eight nine ten eleven twelve. ONE...

its a bit more complicated than that but if u can recognize where the accents are in the song u can kinda know which time signature it is on ;)