Salad mixes being sold on a shelf
When Buying Bagged Greens, Pay Attention To The Bottom Of The Package
To avoid bringing home a bag of half-spoiled salad greens, pick up the bag and inspect it from every angle. Any wilted, slimy leaves should slide to the bottom of the bag.
If you notice any off-colored, visibly wet greens, then pick a different bag. Another visual cue to look for is bag inflation, as decomposing salad greens emit carbon dioxide.
Depending on the bag's material, this isn't a guaranteed indicator, but looking for flatter packaging is always a good idea.