It's quite impossible to tell without closer inspection... In H. diphreutes, the snout is shorter, the breast is more rounded, and the back margin of the anal fin is less angled towards the posterior end of the fish when compared to the very similar H. acuminatus. There is one simple and obvious distinguishing characteristic between the two species, but the truth is that the differences in these traits can vary with the age of the fish, as well as variation between individuals. Some individuals may very clearly belong to one species or another while a few individuals may stymie even the most seasoned fishkeepers.
One easy way to know for sure that you are getting H. diphreutes is to purchase a fish that came from Hawaii, as H. acuminatus does not occur there. Of course, this may be easier said than done. Remember--just because a retailer sells a fish as "the reef safe bannerfish", or "H. diphreutes" or "Hawaiian caught", it may not necessarily be the case.