I don't think you can.
ADOX is a generic mechanism for database schema manipulation. As such, it is made to operate with a variety of providers, most of which don't have an IDENTITY attribute (e.g Oracle and JET).
It may be a provider-specific property of the OLEDB for SQL Server provider, but I couldn't find any mention of it in any documentation I had.
You might try the SQL-DMO objects as an alternative. That does expose an explicit identity property of a (table) column object.
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