Above all, lettuce should be fresh and crisp. It is easy to spot wilted greens. Watch out for limp, withered leaves that have brown or yellow edges, or dark or slimy spots. Once the greens have passed their prime, there is no way to restore them to crisp freshness.
Lettuce should be displayed at the markets under refrigeration, or on ice because it is quite perishable.
Try to choose lettuce with healthy outer leaves—these are likely to be the most nutritious part of the green, containing much more beta carotene a bit more vitamin C than the pale inner leaves. Unfortunately, the outer leaves are usually the most damaged part of the head, but from a nutritional standpoint, it’s best to salvage as many as you can.
Any type of head lettuce should be symmetrically shaped. Choose a head with its dark green outer leaves intact and that’s healthy looking. Avoid overly large heads of romaine, which may have tough, fibrous leaves.
Iceberg lettuce should be compact and firm, yet springy. Very hard heads may be overmature and bitter. The stem end of a head of iceberg lettuce may look brown. This discoloration is the natural result of harvesting and does not indicate damage. If the head is not wrapped, sniff the stem end. It should smell slightly sweet, not bitter.