The problem with identifying Acros definitively is you need a microscope, a library of ID keys and boiling acid to get that pesky tissue out of the way. I think, "well uhmm its an Acro" is realistic. Borneman has an article on ReefCentral where he posts pic of 15 acro skeletons and asks, "Which is the A. millepora?" Its harder than it looks. Especially considering how many "milleporas" are floating around this hobby. Plus you get into different growth forms of the same species depending on light intensity, flow, etc.

I would suggest Veros's "Corals of the World" to ID it. It is grouped by relative relation, so similar corals are placed close together, opposed to alphabetically. Its pretty good for hobbyists, even though it has a disclaimer in the beginning that its not supposed to be used as a taxonomic key. But it can get you relatively close.