You’ll all recall that the identification of a STEMI in the setting of an old LBBB can be made using the Sgarbossa criteria, described in 1996 \cite{Sgarbossa_1996}. The significance of a presumed new LBBB seems to have been overstated in the past, with this now being removed from the 2013 ACC/AHA STEMI guidelines as an indication for PCI due to poor accuracy \cite{O_Gara_2013}. It is suggested that the Sgarbossa criteria can be used to evaluate the likelihood of STEMI in any LBBB, presumed new or old.
Life in the Fast Fane, as in most things, has a great summary on this (http://lifeinthefastlane.com/ecg-library/basics/sgarbossa/), and the crux of the matter is that you can call a STEMI in the presence of a LBBB (be it old or presumed new) if you score ≥ 3 points (Figure \ref{fig:criteria}):
Concordant ST elevation > 1mm in leads with a positive QRS complex (5 points)
Concordant ST depression > 1 mm in V1-V3 (3 points)
Excessively discordant ST elevation > 5 mm in at least two leads with a negative QRS complex (2 points each)