Detecting slope angle

There are some ways. But they depend on how you made a collider for a stairs. For example, if it is simply inclined plane (write on CSharp):

  private float myAng = 0.0f;

  void Update() {
   //Here move your controller and fr slope angle:
   if(myAng < 30) {
    //Simple move animation
   } else {
    //Stairs animation
   }
  }

  void OnControllerColliderHit(ControllerColliderHit hit) {
   myAng = Vector3.Angle(Vector3.up, hit.normal); //Calc angle between normal and character
  }

And second. If you designated each step of a stair, it is necessary to calculate pregoing and current positions. And then also to calculate a angle. But between character and its movement. I hope that it will help you.