When you mouse over an object on your ship that occupies cargo, like a commodity or armor, it highlights the amount of room it takes up on the ship's little cargo bar. However this does not apply to new cargo changes, things like turrets, mounted codenames, and heavy CM launchers.
Also in game there is no indication of how much cargo each of these objects would occupy, I know it is on the forums but it would be convenient if it was listed on the item as well like how the armors list it.... though I hesitate to mention that because it would mean altering the descriptions for every piece of equipment that doesn't go on a fighter.