Most sites will state if they are Joyetech. If they're not Joyetech, chances are it's not a genuine eGo, but a clone. Some good clones are the Riva and KGO.
The previous poster mentioned the eGo C Twist
batteries. That's the way to go, as they are variable voltage. Of course, besides the
batteries, you'll need a charger, some cartomizers, and liquid for the cartos. Get two batteries.