I have the same question. Just got an x3-02. The usual code of "*#62209526#" does not work.
Look at the label/sticker under the battery. It is probably printed there. Possibly also on a sticker on the original sales box.
It can also be seen in the phone in case you lost your box, by going to Settings>> Connectivity>>WLAN>>Available WLAN>> touch and hold the found WLAN>> select "Details"