This is what I was told - Make sure that they have vibrant colors (of course all bettas vary in color but you don't want a dull fish because sick fish tend to lose color), make sure there are no white, brown, or black growths on the fish, make sure that the fins are intact and none are ripped...of course if you want to 'rescue' a sick fish that would be noble of you, but a lot more work.
Skittles was really active in his "cup" at Petco and he is swimming all around his 10G tank and is really active and healthy. As everyone said, make sure their fins are not torn, they have no gray bumps or look pale and sickly etc. Get the brightest, most active one.
And unless you are into the rescuing thing (which just makes them more money and encourages them to mistreat their animals), if most of the bettas look like crap then don't buy the one that doesn't. Complain and walk out.
Personally, I'd buy one online direct from the breeder. I don't like to support places like Petco in their betta cruelty. The ones you get from a reputable breeder will be prettier, healthier and younger, which means they'll live longer. I often browse around aquabid for the USA breeders.
I got my beautiful Sparkle at the Petsmart...I could not tell what he really looked like in that blue water...but when I got him home and into his previously cycled tank ..WOW! What a beauty he is! I did not want to get another Betta..I was buying supplies..but every time I walked by him he went "nuts" in his little cup....so now he lives in my house in his own 6 gallon tank!
I've been looking at the bettas in Petsmart when I visit*. I've noticed that a lot of the bettas have dull black eyes, as in completely black. They don't even look like eyes anymore. I'm assuming this is probably a bad thing. Is it a condition that will clear in a good environment or is this most likely permanent damage?
*My LFS isn't a full service store unfortunately. Hardware they got, fish they got, decorations not so much.
I can't say for sure about the eyes, but it doesn't sound good to me. You can see the pupils of my betta's eyes, and around them they are a healthy light red/pale blue color. They match the color on his face. I think that he has amazingly cute eyes, especially when he looks up hoping for a bit of food! That's just my experience. Good luck.