Brisket is naturally tough, requiring many hours of cooking at low heat to become tender. It makes sense to me that the tenderer the brisket when you start, the better it will be when it is finished. For this reason, I recommend taking a few precautions when choosing the brisket you will take home with you.
First and foremost, if you are fortunate enough to find briskets wrapped in plastic alone, you can lay them, one at a time, across the side of your hand to see which has the most bend. The most flexible brisket is going to be the tenderest. Some briskets are sold on Styrofoam trays, which prevent the bend test from being performed.