Alternative to OnDrawGizmos

CharacterNavigationEditor.cs

using UnityEngine;
using UnityEditor;

// http://unity3d.com/learn/tutorials/modules/intermediate/unity-tips-and-tricks-grab-bag

[CustomEditor(typeof(CharacterNavigation))]
public class CharacterNavigationEditor : Editor
{
    static bool visualizePath = false;

    public override void OnInspectorGUI()
    {
        DrawDefaultInspector();

        visualizePath = EditorGUILayout.Toggle("Visualize Path", visualizePath);
    }

    [DrawGizmo(GizmoType.Selected | GizmoType.NonSelected)]
    static void DrawLinesForNavAgent(NavMeshAgent agent, GizmoType gizmoType)
    {
        if (!visualizePath || agent.path == null)
            return;

        Gizmos.color = Color.red;
        Vector3[] points = agent.path.corners;
        for (int i = 0; i < points.Length - 1; i++)
        {
            Gizmos.DrawLine(points[i], points[i + 1]);
        }
    }   
}