Leif, it's really tough to tell. You have to actually count the spines on the dorsal fins to really tell and even then it's tough.
Normally onyx percs have some orange even between the stripes. Those pictured fish are some nice looking adult black and white ocellaris because they are completely black other than the white stripes. It took my black and white ocellaris about two years to get the full black.
They are great looking fish. If I didn't have two pairs already, I'd be wanting them hehe.