I wouldn't call it fake, there is wild caught and there is tank raised.
Wild caught fish generally are more skiddish, sometimes have more issues with disease or parasite (usually dealt with by the importer/exporter), and some species are more picky as to what they eat and don't eat.
So getting tank raised fish is actually better IMO.
But yeah, I mean there's no way to tell the difference unless you actually saw their paperwork tracing the fish back to the people that caught them.