how to detect OnTriggerEnter on ANOTHER gameObject

Hey future me and other me's, you can use those unity events!

On the other GameObject:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Events;
 
 public class OnTriggerStayEvent : MonoBehaviour
 {
     public UnityEvent<Collider> onTriggerEnter;
 
     void OnTriggerEnter(Collider col)
     {
         if(onTriggerEnter != null) onTriggerEnter.Invoke(col);
     }
 }

On this GameObject:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Example : MonoBehaviour
 {    
     public OnTriggerEnterEvent onTriggerEnterEvent;
   
     void OnEnable()
     {
         onTriggerEnterEvent.onTriggerEnter.AddListener(OnTheOtherTriggerEnterMethod);
     }
 
     void OnDisable()
     {
         onTriggerEnterEvent.onTriggerEnter.RemoveListener(OnTheOtherTriggerEnterMethod);
     }
 
 
     void OnTheOtherTriggerEnterMethod(Collider col)
     {
         if(col.gameObject.tag == "Enemy")
             Debug.Log("Enemy entered the trigger!");
     }
 }

You can't. OnTriggerEnter() messages are sent to the trigger collider and the rigidbody (or the collider if there is no rigidbody) that touches the trigger only - you can't subscribe to them from a 3rd party script. http://docs.unity3d.com/ScriptReference/MonoBehaviour.OnTriggerEnter.html

You can of course create a global collision manager, but each object will have to be responsible for updating it with their own collisions. Assuming all your sub-objects are created from the same prefab, or derived from the same parent class, you simply attach your collision script there.

for those who still need this you can do this.

on the parent (weapon holder) script you can create a method like this. the method will take a collider and then we can assign it to anything.

 public class WeaponsController : MonoBehaviour
 {

 //we would drag and drop the weapon in the inspector
 public GameObject weapon;

 private float Damage;
 private Collider EnemyCollider;  
 private bool enemyHit;

      public void Start()
      { 
            //on our weapon we may have a script called  "Sword" that contains all of the sword 
            //information, such as "Damage"

            Damage = weapon.GetComponent<Sword>().Damage;
      }

     public void EnemyHit(Collider other)
     {

             EnemyCollider = other;

             //if we have a health script on our enemy we may want to damage the enemy
             //so lets say we have a health script
             //we could call the take damage function on the enemy 
             //that was hit

             EnemyCollider.GameObject.GetComponent<Health>().TakeDamage(Damage);
             Debug.Log("Enemy Hit");
             enemyHit = true;

             // here we can take the collider that our weapon has sent us and we can assign
             // it to a new variable so that we can make any changes.

            
         }
     }

then on the weapon you can create a script and do this. you can check for collision and then call the function from your parent script and use the collider that was detected as the collider in the method you created.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class WeaponCollisionDetection : MonoBehaviour
 {
    public WeaponsController WC;

    public void OnTriggerEnter(Collider other) 
   {
 
     if (other.transform.tag == "Enemy")
     {
 
         WC.EnemyHit(other);
 
     }
 
   }
 }

this will allow you to grab what ever your weapon hit and then do what ever to it, such as damage it.