Hard to tell. I know on their higher end hardshells, Arc is still producing them in Canada (e.g. the Alpha series etc). I would call up their customer service and confirm with them where their Theta is made.
It looks pretty legit to me based on what I can see in the pics. Check the seam taping...Arc does such a clean job with their micro-tape that it's hard for other's to fake. The taping should be very narrow. Also, check what the inner pocket material is made of (the one with the Greek Alphabet). On my alpha sl, my pocket is made of a stretchy lycra type material.
Hope you get a sweet deal, other wise come back here and we'll be happy to give you advice on other shells. I personally have ditched goretex for eVent fabric, and therefore ditched Arc for Rab