A healthy plant is one that is fairly full, has a dark green color to the leaves, and is well branched (if it normally branches). A long, tall plant that does not have a good foliage canopy is usually one that has been kept under low light for too long, has had problems in growing, or does not have the root system that it needs. An exception to this may be very large Dieffenbachia or Dracaena. These plants tend to lose some of their lower leaves as they get older. However, you still want to look for plants that are full of leaves and have plenty of fresh growth.
One thing to avoid is a plant that has tender, young growth. If such a plant is placed in a darkened room, the new tender growth will be the first to deteriorate. Make sure that the plant is not a very old one, because this can cause problems.
To better serve your customer, you should acquire the knack of spotting heal thy, good quality plants. To a lot of people, the biggest is the best. There are several clues to a plant's health, but size is not always one of them; shape is, however. Choose plants that are short and stocky rather than long and leggy. Leaf color is another quality to check. Look for lush foliage instead of pale, faded leaves. Remember, too, that leaves without insect bites are not necessarily pest free. Check the stems and underside of the foliage for brown spots and in sect webs. Look for white or yellow dots. Become a plant critic. Make sure the plants you buy and sell are healthy ones!