Of course, after adding local ads to your database, you want to have a look at what the page looks like for a visitor from a different country. It took me some time to figure out a way. But I found a good method for getting a preview for a different country. In openx version 2.6 you can edit the file /plugins/geotargeting/GeoIP/GeoIP.delivery.php at line 141. At the end of the OA_Geo_GeoIP_unpackCookie function, add something like

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    if ($_SERVER['REMOTE_ADDR'] == 'HERE.IS.YOUR.IP') { // insert your IP
        $aGeoInfo['country_code'] = 'ES'; // insert country code
    }
    return $aGeoInfo;

That’s all, no proxies or messing with your tags or sites required. I hope a similar feature will be added to the plugin sometime in the future. Oh, and I’m not quite sure what would happen if cookies are disabled, but who cares, this is just for testing.