http://docs.unity3d.com/Documentation/Components/gui-ExtendingEditor.html

example

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;
[CustomEditor(typeof(MyClass))]
public class MyClassEditor: Editor
{
    public override void OnInspectorGUI()
    {
        GUI.color = Color.yellow;
        EditorGUILayout.HelpBox(target.GetType().ToString(), MessageType.Warning, true);
        GUI.color = Color.white;
    }
}

edit

first you need a MonoBehaviour (AKA Component, Script, Class) for object. let's sat i have a Bullet Component that needs bullet prefab attached and on starts it instantiates one instance of it and gives it a speed 10 to forward. to give a speed we need first to check if Rigidbody component is attached to bullet.

Bullet.cs

using UnityEngine;
using System.Collections;
public class Bullet : MonoBehaviour
{
    public GameObject bulletGameObject;
    void Start()
    {
        if (bulletGameObject.rigidbody)
        {
            GameObject newBullet = Instantiate(bulletGameObject) as GameObject;
            newBullet.rigidbody.velocity = newBullet.transform.forward * 10f;
        }
    }
}

you already can attach this script to any GameObject in scene and use.

but now we need a custom editor with a warning message about rigidbody. let's do it.

this script first associated with Bullet Component - it means this is inspector for Bullet component only (line 5). now, in method OnInspectorGUI (that draws inspector) we write our own controls. first we take a 'target' (this is always a Component we edit with inspector) as draw a field to select GameObject (same as simple public reference in Component).

and here's the main part.

we check if selected gameObject has a rigidbody component, and draw info 'OK' or error 'Not OK' message

BulletInspector.cs

using UnityEngine; using UnityEditor; using System.Collections; using System.Collections.Generic; [CustomEditor(typeof(Bullet))] public class BulletInspector : Editor { public override void OnInspectorGUI() { Bullet b = (Bullet)target; b.bulletGameObject = EditorGUILayout.ObjectField("bullet GO", b.bulletGameObject, typeof(GameObject), true) as GameObject; if (b.bulletGameObject) { if (!b.bulletGameObject.rigidbody) { GUI.color = new Color(1f, 0.5f, 0.5f, 1f); EditorGUILayout.HelpBox("object must have a rigidbody component", MessageType.Error, true); } else { GUI.color = new Color(0.5f, 1f, 0.5f, 1f); EditorGUILayout.HelpBox("object have a rigidbody component", MessageType.Info, true); } GUI.color = Color.white; }

 }

}

IMPORTANT

don't forget to place last script and all other editor scripts in 'Editor' folder. or else your project won't compile.

 var bullet : gameObject;
     ___________________
     | the bullet       | 
     | need a rigidbody |
     | or not work      |
     |__________________|