For a phaeleonopsis orchid, check the leaves.
If they are floppy/super bendy or wrinkled, that's a sick orchid. Don't buy it no matter how pretty the blooms are unless you don't mind chucking it out after the blooms fall off. Also, the blooms won't last as long if it's sick. See the roots hanging out? That's a good sign!
For orchids with grass-like leaves, stay away from wrinkled leaves as that means there's something wrong with the roots (probably root rot) and the plant is not getting enough moisture:
Most other orchids you will find (unless you're seeking out the unusual) have pseudobulbs.
This is what you might think of as a stem on a regular plant, but it's not quite the same. What these are are reservoirs the plant creates to hold moisture and nutrients. They should be plump, no dark/black parts (check the base) and minimal wrinkles.
If there's just the bulb, and no leaf, don't worry about that. It's normal. It just means the bulb is 'full' and doesn't need the leaf anymore:
So, remember the orchids I'm rehabbing? Like the one with no root system:
The reason this orchid will pull through, if I take care of it properly, is because it will live off the energy stored in the pseudobulbs to send out a new pseudobulb, and from there, new roots like the cattleya I have with crown rot:
Here's another orchid I'm rehabbing that looks pretty dead, right? The pseudobulbs are shrivelled, the leaves are brown/dying, and it's being held up by several supports:
For sick orchids like this, I keep a spray bottle of water and I mist the top of the bark a couple times a week. Keep the spray on 'mist' though, as you don't want to douse the already-sick plant in water and give it crown rot on top of everything...
Okay, that's the end of my orchid-nerd-fest.