How to make a receptacle box flush with wall after it has been installed?
That is an adjustable box. 

Along the right vertical side you see that screw? That screw will adjust the box in and out. But since someone painted/caulked it in you will need to free it up all around the drywall edge.

I have to say though, whoever installed it in the first place did a really shitty job. It should sit even all the way around.