How do you use your Ability Radius?

What is the keyboard key to see the radius of your ability? I have attached a picture showing Rhux, LCS pro, using the radius to find out if his spear can reach a target.

Would thoroughly appreciate your tips on how to do this.

EDIT: When not using Smartcast, you can see a long line of where the spear would go. What I want to know is how to get the radius of where the spear can go.