How do you tell between a grouper and a cod? If you've been eating them regularly, you'd be able to tell the difference right? Think again. The thing with fish is, once you fillet it and get rid of its characteristics, it can be very hard to tell what from what.

The Smithsonian Mag has a piece on LeeAnn Applewhite of Applied Food Technologies. The company specializes in identifying fish through their DNA. Applewhite has actually found that some lobster bisques at restaurants are actually shrimp. And some common fish fakeout is just Vietnamese catfish masquerading as cod. According to the Smithsonian:
To identify a piece of fish, AFT staff slice off a tiny sample from a fillet, heat it up to break down the tissue and open up its cells, and spin it in a centrifuge to extract the DNA
The company will then look at a few genes to reveal the difference. The pictures above and below are from a 2011 study at Oceana, escolar and atlantic cod, nile perch and grouper, swordfish and mako shark, red snapper and rockfish and farmed salmon and wild salmon.