You can't - the bar codes on beer labels don't change daily/per batch. The date is stamped on the glass itself, near the shoulder of the bottle (often difficult to read and/or incomplete- you'll probably have to check more than one bottle in a sixpack) and the "Best before*" type date is stamped on the side of the cases. *Forget Troegs' exact wording and if the shelf life period is 3 or 4 months (my empty bottles and case have all been hauled away by the recycling truck).
The best by date on the case is 4 months from the bottle date, which can be found right where jesskidden said. Some bottles are entirely illegible, and some have an incomplete code, but most have one that can be deciphered.
