SPField.CanBeDeleted property
SharePoint 2013
Gets a Boolean value that indicates whether the field can be deleted.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
This property returns the value of the AllowDeletion property if it has a value. If AllowDeletion does not have a value, CanBeDeleted returns false if either the FromBaseType property or the Sealed property return true.