Look for spots, torn fins, no movement, stress stripes without being moved, If they don't have color at all, Etc.
Then you have an unhealthy one.

 

i agree except for the color part, sometimes they have lack of color due to crappy food. i have had bettas in the past that were butt ugly and have some TLC and good food, they within days blossomed into some beautifully colored bettas!

 

and play with each one you're interested in by turns
if they don't play there may be a good reason, if they play too much there may be a bad reason

be careful not to stress them in the cups
(odd, my pristella are trying to school through the back of the tank just now)

Attentive instead of attacking almost always means a good pet.

 

Do you have a Petco nearby? They have more variety in terms of finnage... halfmoon, plakats, veiltails, crowntails...
You can call the store and ask them when they receive their betta shipments and go a day or two afterwards because sometimes it takes that long for bettas to make it to the shelves.
If you're wanting a healty betta, pick the one that seems more attentive and even flares when you put him next to another betta.
You want their fins to be straight free of black areas (unless is a black fish) and not curled... these are all posibles signs of bad water quality.
Also, the water should be as clean as possible... no gunk on the bottom and no clowdiness. Don't let yourself be fooled by a betta that has a bubblenest... I've seen bettas in pretty bad condition at petstores blowing bubbles so you shouldn't reply just on this as indication of a healty betta.
At Petsmart and Walmart is harder to see the condition and colors of the fish because of that blue water and you might have a hard time getting them to flare because they are used to sitting next to other bettas.

 

Do you have a Petco nearby? They have more variety in terms of finnage...

I do have a Petco nearby. Two, in fact, but one store is significantly smaller than the Petsmart and has a much smaller selection of Bettas. The other Petco I haven't checked yet so I will call them and ask. Thanks, and thanks for your other tips!

AMETHYST

 

Like Vilmarisv said, you'll probably get the best bettas right after they're shipped to your store. So, call and go the the store once they get their next shipment.

a healthy betta is active, and responsive. He will have his tail unclamped and flowy. He'll move around in his cup and have good colors.

A sick betta is either bloated or too skinny. He's unresponsive. He might have clouded eyes, rotty clamped fins, stress stripes, or pineconing. He will simply sit in his cup, with little to no wiggling or movement. His colors are probably faded and pale.

 

I take a flashlight, usually two.

I've got an ultraviolet frequency LED flashlight I use for work (and for play, checking circuit boards for coating) and use it to check slime coat and water bacteria levels. You'll get a heavy green response if the water is full of bacteria.

They're expensive tho... if you have a Batteries+ in your area they often have $10.00 one-battery counter flashlights. Handy little things for checking out your little options at the store, just shine up from below or on the plastic lid for general light. Betta can't blink so don't hurt em by direct, expect lots of flaring to happen when you shine on them.

 

I've got an ultraviolet frequency LED flashlight I use for work (and for play, checking circuit boards for coating) and use it to check slime coat and water bacteria levels. You'll get a heavy green response if the water is full of bacteria.

I have a UV flashlight designed to check for urine in carpets (I have dogs :lol. Would that work? That's a great tip, so thanks!

AMETHYST

 

should, yup. Just be careful not to freak people out with it.

a good slime-coat, gill damage, bacterial deposits all show up with the uv.