How I'm doing it:
Debug.DrawRay (transform.position, Vector3.forward*100 , Color.red);
So debug is a class that contains that governs all the debugging functions.
DrawRay is the function we want to draw the line.
Then the next part of the code goes like:
(Position of where the ray will start which is a Vector3, direction of ray and how long, and then the color of it)
For the direction of a ray it can go 6 ways. (up, down, left, right, forward, backward)
Vector3.forward is just shortcut instead of saying Vector3(0,0,1)
And for those who aren't familiar with Vector3, its just 3 coordinates to define a object in 3d space. It goes (x position, y position, z position). x is left and right. y is up and down, and z is forward and backward.
Colors you can either make by using the class variables or by using one of it's predefined colors. In the colors class, it's predefined variables are : red, green, blue, white, black, yellow, cyan, magenta, grey, and clear. Read more on the colors class here.