This is visible from the report at Reports > Material > Stock Level > Quantity in Stock.
After raising a New Purchase Order the Quantity on Order stock level will be updated.
It is also possible to view and edit this in the Edit Material page. To do this, from the Main Menu select Set Up > Material and click on Edit next to the Material you wish to see.
You should see a page similar to
In the above example the Quantity on Order (highlighted above) displays the quantity that has been ordered (through a Purchase Order) but not yet received.