Well, when you select the Soul Gem, it tells you if it's empty or it has a soul in it. It even tells you what soul it holds (lesser, grand, black, etc)..
Yet if you aren't talking about Elder Scrolls' soul gems, then you probobly need to set a variable when capturing the soul, and then check the variable.
If you have only "full" and "empty", then a boolean would be sufficient, and if you have more soul-types, then you need to use integer.
Or perhaps you could use several items - 1 for each state of the soul gem. And then check the item type.