no one copies the altia kit.... no one. lol, it'd be kinda silly, esp when the factory option was so cheap compared to aftermarket according to the catalog.
if the r34 has an altia option kit on it, then its most probably genuine. there's plenty of genuine ones going cheap, and most of them have had them since day 1.
fibreglass kits are usually unpainted on the inside, a photo of the inside of the bumper will tell you pretty easily what it is.
failing that, just kick it. if it's fibreglass... run. if not, buy.