I have an unopened bag of garden salad (lettuce, carrots and red cabbage) That is 9 days past the "Best if used by" date. I haven't opened it yet. It still seems crisp when I squeeze it and I don't see any wilting.
Many salad packages contain freshness preservatives. That is, if the salad remains inside the original packaging, it will maintain its freshness. Regardless of the "best by" date, you should look for crispness of the greens. If the greens are crisp and smell fresh, they are good to go.