Is there a way to see how SQL Server 2012 defines new identities?
SELECT INTO has two phases (not visible in execution plans).

First, it creates a table that matches the metadata of the query used to create it. This happens in a system transaction, so the (empty) created table will continue to exist even if the SELECT INTO is wrapped in a user transaction that is rolled back. At the end of the first phase, we have an empty table that includes a column named A_ID with the identity property.

The second phase performs an insert into the table the first phase created. This insert is done with identity_insert semantics, so the identity values from the source table end up in the destination, unchanged. The highest value actually inserted is set as the last value used. You can use IDENT_CURRENT or sys.indentity_columns to see it.

Unrelated, but semi-interesting fact: There is a small window between creating the table and starting to perform the insert, where the source table in the query is not protected by a schema stability lock. If a concurrent process happens to change the schema of the source table (after the target has been created, but before the insert starts), error 539 is raised: 


  Schema changed after the target table was created. Rerun the Select Into query.