It's hard to get a quick overview on which buildings can use rain water and are using rainwater and also what kind of rain water.
Where the alt key gives you a nice overview on what workers are in what buildings an extra key that shows all the buildings and their possible types of useable rain water and if they are currently connected would be a nice addition.
Hey there! There is an overlay in the game that does this. You can invoke it by holding TAB. It will show you water types, where engines are installed, and the level at which they are using rainwater.