Several companies have distribution deals set up with Shenzhen Joye Technology Co., Ltd, the original designer of the eGo. The Tornado from Totally Wicked is an example of a renamed authentic eGo. Cignot is a dedicated
joyetech reseller in the US. Many of the US suppliers are reselling authentic merchandise. Find a reputable ECF supplier here on the forums, once you've verified ECF members opinions on a vendor, call them up and ask.
joyetech is the parent company of Ovale in the US and Canada, in the UK it's partnered with Janty who influenced design of the eGo. When in doubt you could always just stick to the partners although they usually charge a little more.