They are marked as such, and appear under the heading of the generic unit they replace. Note that many units are unique to certain civilizations that is, they replace a certain generic unit and will only be available when you play with a certain civilization. The Brave New World expansion added 20 new units, including the new Barbarian unique unit Hand-Axe. The Gods & Kings expansion added 27 new units, including the Composite Bowman, Galleass, and Landship. Here is the full list of units in Civ5, as of the Brave New World expansion.
