how is cold-potato done?

If I peer with network X in cities A and B, and receive the same route in both cities with an AS-path of X, how do I know which city to use for an exit? I can understand how if X uses communities to tag the geographic origin of the traffic, but I'm not aware of many networks that do this. Lots of networks claim to use cold-potato routing though, so how do they do it? Ralph Doncaster principal, IStop.com