Existence

If material to the financial statements, all inventory, whether manufactured (from bought-in raw materials and/or components) will be covered by physical count procedures. (Work in progress might be ignored if immaterial.) If it’s not on the clients premises (warehouse/factory) to physically inspect – for example, raw materials are stored in a warehouse owned by a third party – the auditor may still be able to physically inspect it (e.g. accompanied by client’s staff). In relatively rare cases, if its not possible to physically inspect the goods (e.g. if imported materials or goods are held “in customs” – pending payment of duties or tariffs), the auditor may obtain external confirmation.

If you meant valuation assertion, rather than existence, then yes – you would verify costs of raw materials to purchase invoices, labour costs to job sheets etc and production overheads to cost records and calculations. The same principle would apply if you were looking at a self-constructed non-current asset (e.g. a building).