How to test something in Multi-Threaded Environment using Java

• How do I test something like this in multithreaded environment. I know it's gonna fail, cause this code is not thread-safe. I just wanna know how can i prove it? Creating bunch of threads and trying to add with those different threads?

public class Response_Unit_Manager {

private static HashMap<String, Response_Unit> Response_Unit_DB =
        new HashMap<> ();

/**
 * 
 * This subprogram adds a new Response_Unit to the data store.  The
 * new response unit must be valid Response_Unit object and it's ID must be 
 * unique (i.e., must not already exist in the data store.
 * 
 * Exceptions Thrown:  Null_Object_Exception
 */
public static void Add_Response_Unit (Response_Unit New_Unit) 
        throws Null_Object_Exception, Duplicate_Item_Exception {
    String Unit_ID = New_Unit.Unit_ID ();

    if (New_Unit == null)
        throw new Null_Object_Exception ();

    else if (Response_Unit_Exists (Unit_ID))
        throw new Duplicate_Item_Exception (Unit_ID);

    else
        Response_Unit_DB.put (Unit_ID, New_Unit);
} //end Add_Response_Unit