You can pass the mouse over the region, the tooltip will indicates how much supplies chits the region produces. Each supply chit gives 5 food points and 2 ammos points to the units.
Check also the tooltip over the units, in the regiment panel (bottom center), it gives the stocks of each unit. Finally you can click on each company, the small NATO symbol to the bottom right, you see in the detail window the supply usage and stock of this element.
Forgot also, you have the supply gauge on the regiment panel.
And don't forget supply wagons either
