Check the disks for Asian characters, for info see here:
Linkypoo
Even some of the Voyager ones are dodgy.
From memory, the difference is the genuines have a transparent-coloured plastic box while the copies are non-translucent.
Saw photo's of the difference on Amazon somewhere, but now can't find it.
Found this one tho of the packaging I'm talking about. Current run of DVD's are repackaged so look very different.
If a seller claims they're region 4 they're probably okay.