The Star had a story on fish mislabeling a while ago. One of the most common is tilapia sold as snapper (I'd been seeing this one at cheap sushi joints long before The Star story).
Another one that they came across, but I'd never seen until this weekend, is basa sold as sole.
Looking at it before buying it I couldn't help thinking "that's very weird looking dover sole" but at $5.99lb and at a major store I figured okay I'll buy it since it's going in stew and basically all I want is the least expensive fish in the store.
So when I was getting ready to put the fish in the stew I had a good quick feel and inspection:
1) veiny, sort of cartiliginous feel/look
2) reddish streaking and slight brownish tint to the meat
3) division of muscle/meat sections (don't know a better way to call it - the individual fillet parts?) not similar to sole
4) too thick/meaty
5) doesn't smell like sole
After cooking I knew for certain it is not sole by the taste, texture and appearance - the texture and way the fish separates is nothing like sole, the taste is something like catfish and appearance is again slightly brownish and thick/meaty.
This is only one consumer's experience, but if I've encountered fish mislabeling at one of the more major stores then others probably have too.
Buyer be warned - if it doesn't look like top grade tuna trust your instinct it isn't. If it doesn't look like sole it isn't. If it doesn't look like real red snapper it isn't, etc. etc.