Check to see that the plant is firm in the pot. Orchids that are shaky likely have recently been re-potted or may have a bad root system. A re-potted orchid is not necessarily a bad one, but you will want to check the mix. Sphagnum moss is frequently used to pot orchids in for shipping. This mix does not dry out and can cause the plant's roots to rot quickly. You will want to replace it with a mix specifically formulated for orchids.
Observe the leaves, which should be firm. Limp and crinkled leaves indicate the plant may have been exposed to excessively cold temperatures, or that it has not been watered correctly. Also be mindful of brown spots on the leaves. This may indicate exposure to too much sunlight or high temperatures.
Pay attention to the plant's pseudobulb, which must be fat and plump. Do not choose orchids with shriveled pseudobulbs. The pseudobulb is the thickened, bulblike, fleshy portion of the plant's stem.
Be mindful of the flowers. Limp flowers and missing buds are signs the plant has been stressed. It will most likely drop all of its flowers within a few days, regardless of what you do to salvage it.
Avoid an orchid plant if you notice a sticky substance on its leaves. This means the plant likely has bugs.