Look under the leaves, if you see any sign of a white mold or a slimy brown leaf, put the pack back on the bench. If you see several packs in different parts of the shopping area with the same symptoms - go to another retail outlet. The key sign here is to see how you feel when you walk into the retail greenhouse. If it is hot and humid and uncomfortable to be inside the greenhouse area - there is likely disease present. Good growers have very high rates of ventilation to keep the plants dry and healthy.
Poor growers have too hot greenhouses with too small or non-existent fans.
Look for bugs. Oh boy, do they like tender bedding plants. A tip here is to casually wander over and check out the green peppers. Green peppers are aphid magnets. If the peppers have aphids, you can be sure there are aphids elsewhere too.