The New York Times style guide is a handy way to tell: If the Times puts periods between the letters ("A.C.C." for the basketball conference) then it's to be pronounced as individual letters. If the Times puts no periods between the letters ("NATO" for the treaty organization) then it's to be pronounced as a word. That decision is then used to figure out whether there's an "an" in front of it.
The beeb (BBC) works in a similar way. Things that are spelled out are put in all caps ("ACC") and things that are pronounced are put with only a leading capital ("Nato").
Of course, that's just the journalism wonk in me speaking.
