You can call the function GetThemeScrollBarThumbStyle to obtain the type of scroll boxes (also known as "scroll indicators" or "thumbs") being used in the current theme. The ThemeScrollBarThumbStyle constants are available with Appearance Manager 1.1 and later.
enum {
kThemeScrollBarThumbNormal = 0,
kThemeScrollBarThumbProportional = 1
};
typedef UInt16 ThemeScrollBarThumbStyle;