How to detect intersection in meshes?

Hello, I'm making a game where you can instantiate 3d objects on the location of the mouse cursor. I need to know in runtime if a collider or a mesh of an object the script is attached to is "inside" another. This to prevent objects beeing instantiated in eachother. I can't use built-in collisions because they can (seemingly) only notice surface collisions with rigidbodies. OnTriggerEnter doesn't seem to work like I want either. Here's the javascript:

 #pragma strict
 
 var CInfo : Collision;
 var ObjectToCollide : Collider;
 function Start () {
 
 }
 
 function OnTriggerStay (ObjectToCollide) {
     renderer.material.color = Color.red;
 }
 
 function OnTriggerExit (ObjectToCollide) {
     renderer.material.color = Color.white;
 }

What should I do?