At lower levels, it's usually labelled as "Disciples of War", "Disciples of Magic", or "All Classes" - you need to look at the stats to determine which piece of armor is best for you in this scenario. For example, a CNJ will usually want armor that increases their MND stat.

At mid-higher levels, the armor starts becoming specific to your class/job, so instead of "Disciples of War", something like a pair of Aetherial Steel Gauntlets will say "MRD WAR GLA PLD", specifying that the armor is for those two classes and two jobs only. You still want to look at the stats, but this helps eliminate the whole "Okay, could I use these gauntlets as an ARC?" deal - you instantly know whether you need to even consider the gear.

Hope that helps!