Customise Inspector GUI and Scene GUI

LookAtPoint.cs

//C# Example (LookAtPoint.cs)
using UnityEngine;
[ExecuteInEditMode] // Can execute in edit mode even game is not running.
public class LookAtPoint : MonoBehaviour
{
    public Vector3 lookAtPoint = Vector3.zero;

    void Update()
    {
        transform.LookAt(lookAtPoint);
    }
}

LookAtPointEditor.cs

//C# Example (LookAtPointEditor.cs)
using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(LookAtPoint))]         // Target script
[CanEditMultipleObjects]
public class LookAtPointEditor : Editor
{
    SerializedProperty lookAtPoint;

    void OnEnable()
    {
        lookAtPoint = serializedObject.FindProperty("lookAtPoint");     // Get a property in target script. Notice that string "lookAtPoints" presents Vector3 lookAtPoint, not script 'LookAtPoint'.
    }

// Custmise Inspector GUI
    public override void OnInspectorGUI()
    {
        serializedObject.Update();
        
        // Customise, although the same to original format.
        EditorGUILayout.PropertyField(lookAtPoint);
        
        // Add a label field
        if (lookAtPoint.vector3Value.y > (target as LookAtPoint).transform.position.y)
        {
            EditorGUILayout.LabelField("(Above this object)");
        }
        if (lookAtPoint.vector3Value.y < (target as LookAtPoint).transform.position.y)
        {
            EditorGUILayout.LabelField("(Below this object)");
        }
        
            
        serializedObject.ApplyModifiedProperties();
    }

// Customise Scene GUI
    public void OnSceneGUI()
    {
        var t = (target as LookAtPoint);

        EditorGUI.BeginChangeCheck();
        
        // Add a position handle on look at point.
        Vector3 pos = Handles.PositionHandle(t.lookAtPoint, Quaternion.identity);
        
        // Update target object depends on change of look at point
        if (EditorGUI.EndChangeCheck())
        {
            Undo.RecordObject(target, "Move point");
            t.lookAtPoint = pos;
            t.Update();
        }
    }
}