Check for bugs and pests. Flip up the leaves and carefully inspect the axis where the leaves and stems meet. If you see anything that looks like an insect, don't buy the plant. In fact, this may mean that the entire stock in the garden center is infested.
Check for diseases. Danger signs include mushy spots on leaves, black spots, washed out leaves, yellowing, curling and other signs of obvious distress. You don't want to expose your other plants to foreign diseases.
Get a look at the roots. Gently ease the plant from its container (don't worry about hurting it). The roots should be firm and white. If the root ball is a solid mass, however, the plant may be root bound, which isn't good. Alternatively, the plants in nursery centers often aren't fully rooted. If you don't see any roots and the stem is loose in crumbly soil, keep looking.
Inspect the pot. Pots that are slimy with mold or have roots protruding from the drainage holes should be avoided. These plants are likely to be old stock and may be stressed.
Count the buds. If you're buying a flowering plant, don't be seduced by big, showy flowers. Instead, look for tight, unopened buds that promise future blooms. You want to enjoy the flowers at home, not in the garden center!