How to determine if an ALTER statement will be a metadata-only operation O(1) or a size of data operation O(n)?

We are dealing with very large tables in Sql Azure. Size-of-data operations can lock tables for a very long time so we must be careful to run these only on week ends. Metadata operations are quick and we can run them in production at any time. Looking at a given ALTER TABLE statement, how can I determine if this will be a metadata-only operation or a size-of-data operation? As far as I know it's not possible to view an ALTER's execution plan?