Stem should be free of any bends or breaks. Plants that are in bloom when purchased often are under stress. A natural response to stress is to propagate through heavy seed production. Perhaps the best measurement of plant health can be seen in the roots. It is estimated that 80 percent of all plant problems begin with root disorders.
Roots have a pronounced effect on the size and vigor of the plant.
Flip the plant over and look at the roots. If no roots are evident, the plant likely was transplanted recently into its container. If you purchase a one-gallon container, the root ball should fill one gallon. If the roots circle the pot many times, the plant is root bound, a stressful condition that you will want to avoid. Healthy roots should be a creamy whitish color and they should be wet.