Look for signs of freshness.  Iceburg lettuce and romaine, the leaves should be crisp.  Other lettuce will have a softer texture, but leaves should never be wilted.  Look for bright color in most varieties light green.  avoid heads that are hard and lack green color which are signs of over maturity.  These heads sometime develop discoloration in the center of the leaves, the mid ribs, and may have less flavor.  Avoid heads with irregular shapes and hard bumps on top. which indicates the presence of overgrown central stems. Slight discoloration of the outer wrapper leaves will usually not hurt the quality of the lettuce, but serious discoloration or soft decay must be avoided.