The icon for his attacks changes subtly. Also, you can mouseover the icon for charged status attacks like that to see more details on them. Once you can tell the two attacks apart, you just have to use a different strat for each: attacking on one, blocking on the other. I'm sure you can figure out which.
Also, no worries, but consider asking stuff like this in the forums in future. Makes it easier for others with similar questions to find them later.