In Salesforce there are some specific objects like Attachment, RecordType etc. which are 'closed', i.e. we cannot create custom fields on them, in opposite to 'open' objects, e.g. Account, Contact etc.
So the question here is - how to detect from Apex whether we are dealing with 'open' or 'closed' object relying just on provided object name? Really want to avoid hard-coding list of object names)