One easy way to determine if a family Category is Cuttable is by looking in the Object Styles dialog box. If there is a gray box in the ‘Cut’ column, that Category is not cuttable. See image below (from 2012):
For some further info, you can check out these WikiHelp pages:
Cuttable Families – WikiHelp
Non-Cuttable Families – WikiHelp
However, it would seem that the above lists are not exhaustive…
Also, you can check out my previous post about the subject:
What Revit Wants: Cuttable vs Non-cuttable families
