Press f3 and make sure the place is dark.
-.-
I dont know why they removed them, to stop cheating to find dungeons maybe? But I actulally prefer it without the mob IDs as it makes the game more unpredictable. But it would be nice for an option to turn them off or on when pressing f3 for players who whant in single player.
But if you really want to find dungeons, strongholds diamonds or whatever you can always
cartograph your map if you don't mind cheating
But if you want to find them legit, there are some points: More than one of the same mob's noise usually, look out for random cobblestone/mossy cobblestone, dungeons are mostly always at the end of caves and mostly around the mid depths and top of the underground (but this may vary)