Unfortunately, there is no way to tell for sure from the sell by date. That date is applied by the store based on their own policies. What is important is the pack date for that brisket, which is usually stamped on the case. Most stores set the sell by date based on a number of days past the pack date but that can vary by store.
But... In most cases you will be fine for a week. Lots of us regularly wet age briskets in the cryovac for up to 50 days past the pack date. Odds are your brisket isn't much mor than 30 days past the pack date and probably a lot less than that.
You can get some idea of the age by looking at the cryovac package. When freshly packed it is tight to the meat with no bubbled. As the brisket ages it will give off some gas and that will form bubbles. When it is aged about three week the bubbles will be about the size of a dime. 4 weeks, the size of a nickel.
Did any of that help? I'm still drinking my coffee. :-D