When buying plants, look for ones that are short and bushy, instead of a tall plant that has only a few stems. Sometimes long, spindly growth is a sign that the plant has not received enough light, or that it is root bound. When you purchase a plant that is bushy you know that the plant is probably strong.
When buying plants, even bushy ones, you should avoid plants that have leaves that are turning yellow or discolored. This could be assign that the plant has root problems, pests or even a fungus. Also, poke your finger into the container. If it is dry as a bone the roots are probably already damaged, even though the plant itself may look fine.
Lastly, look under the pot. Are there roots coming out the drainage holes? If so, this is a sign that he roots are bound and overcrowded. This could mean problems down the road because the plant may be weakened by the overcrowding. This is especially true for shrubs and trees.