To find out if a curve or surface is closed, look under Geometry info in the Information Window or use the Query edit tool.
StudioTools distinguishes between objects that merely look closed (their start and end points occupy the same space) and objects that have a periodic topology (the control hull coils over itself with several end CVs overlapping) which cannot be opened by simply moving CVs. The Close tool produces the latter. This distinction can become very important when you are applying shaders.