Figuring out what kind of surface I have?

[AndersDeleuran]

Those are actually quite distinct concerns (that may or may not be related depending on your specific cases):

@mary.baker

if given an arbitrary surface, what kind of surface it is

Use the Python function type() to determine explicitly which RhinoCommon geometry type it is.

@mary.baker

Closed in one direction but not the other (a cone or cylinder)?

A closed cylinder/would be a Brep, not any of the surface types. Have a look at this old post by David that explains the different types and how they relate.

@mary.baker

Or is it open in all directions? Or closed in both (like a sphere)?

You can check if a surface is closed using IsClosed, and if a brep is closed/solid using IsSolid.

@mary.baker

Or is the best way to look at its isocurves and try to figure out their curvature?

I’m not sure how that might help, but again probably depends on exactly what it is you’re trying to determine. Anywho, hope this at least helped point in the right direction.

Edit: Sorry think I misunderstood, you probably meant a single surface cone/cylinder that is not capped. Surface.IsClosed should do the trick then. Always good to provide example files.