I have some rules of thumb I follow to figure this out:
Big Picture vs. People Picture
Mature souls are able to take forays into the big picture, and they are very interested in it, but their primary focus is still very much on people and their doings. There is a lot of attachment and worry about goings-on between people. Old souls focus very naturally on the big picture, and they are a lot more philosophical about life in general and about the doings of other people.
Searching Eyes vs. Mellow Eyes
Mature souls have a kind of a searching and hopeful look in their eyes, where old soul eyes look more mellow and relaxed, often with a humorous glint.
Energy and Presentation
In general, the mature soul has a more nervous, coltish energy about them, where an old soul will seem more rounded and mellow. Also, mature souls are more socially acceptable. They may be "counter cultural," trying to change the young soul culture, but they're still very much part of the culture. Old souls, on the other hand, can be total eccentrics without even trying to be or realizing that they are. They are also trying to change things, but in their own special old soul way, which everyone else may think is totally worthless or non-existent. Old souls can come off as strangely lazy or purposeless (not that they are, just that they come off that way to other soul ages), where mature souls always appear to be striving for a goal, though the goals are different than those of young soul goals, obviously.
For me, it's the focus on people vs. big picture that really clinches it when it seems like the person could go either way.