I am very lucky to have a dog (she's about 10 months) who will at least sit by the door and kind of sniff around or just stare at us til we get up and take her out lol BUT, no barking or anything which can be frustrating.
I HAVE heard of a technique where you can train your dog to use a bell to let you know they have to go out. Here's a video of someone having their dog do this... I'm sure there is more information you can find on this somewhere.
Good luck!