Dummy Wire??

[wakeuphicks]

First post on these forums, but nice to know a place exists for me to ask my ignorant home improvement questions.
So here goes - I just installed a digital programmable thermostat - my old one had 6 wires including a orange wire, but my new thermostat only has 4 - I made the 4 connections and everything seems to be working ok, but Ive only tried my heating so far not the A/C. I heard the orange may just be a dummy wire, but is there a way to tell for sure?

[pushkins]

Turn on your AC for a minute, if the AC turns on then your good to go.
It is not uncommon at all for there to be a "spare" wire/s, units can require 6 wires and others not.