When lettuce is bad the lettuce will start changing colors and may turn a brown or yellow color, the bad lettuce will usually be wilted and have a slimy feeling to it and also the bad lettuce may have a sour or bad smell to it.
If the lettuce smells bad is really wilted or slimy or has changed colors then throw it out and avoid getting sick.
It's OK to eat expired lettuce as long as the lettuce is not actually spoiled or has gone bad.
The expiration date on the lettuce does not mean that the lettuce is actually bad or unsafe to eat.
As long as the lettuce is not wilted, does not smell bad or slimy then the expired lettuce is okay to eat.
I have eaten expired lettuce before and I've never gotten sick.
The expiration date on most food does not mean that the food is actually bad or unsafe to eat.
Just examine the food for spoilage and if it is indeed spoiled then throw the food out but you don't have to throw food out just because it's expired.