I believe the person who creates a leaderboard starts out following it by default, but they could unfollow it if they so choose.

I don’t think there’s any way to see a list of who’s following your game. You could just make a forum post in the game’s forum and ask the followers to respond.

Pear is correct, and to add to their suggestion, followers are notified by default of new forum threads, so if there are new followers they'll probably respond

You can see how many people follow a game by checking the statistics tab on the sidebar, but you can't tell who follows the game or not.