Hashtable.ContainsValue  

// Searchs for a value on the hash table

var h : Hashtable;
h = new Hashtable();
h.Add("1","one");
h.Add("2","two");
if(h.ContainsValue("one")) {
    Debug.Log("Value 'one', has been found!");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Hashtable h;
    void Example() {
        h = new Hashtable();
        h.Add("1", "one");
        h.Add("2", "two");
        if (h.ContainsValue("one"))
            Debug.Log("Value 'one', has been found!");
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public h as Hashtable

    def Example():
        h = Hashtable()
        h.Add('1', 'one')
        h.Add('2', 'two')
        if h.ContainsValue('one'):
            Debug.Log('Value \'one\', has been found!')