The manufacturing order has different status according to the availability of the raw materials. (Awaiting Raw Materials -> Ready to Produce)

So, OpenERP computes automatically the availability of the raw materials and change the status of manufacturing orders accordingly, when all raw materials are available. It do the reservation of the products so that other order would not use them.

You can also check per manufacturing order line by clicking on the raw material from the manufacturing order. You can also check the status per product/line: Awaiting another move (products may be available but should arrive from another location) -> waiting availability (products are not available) -> available.

There is no warning but, usually, the workers only work on the list of MO in "Ready to Produce" state. If the manufacturing order is "Awaiting Raw Materials", the user has to manually force the reservation of the products before starting the manufacturing order so it avoids launching MO when raw materials are not ready.