There are a number of places to look.
Firstly, during the BIOS POST routine the BIOS checks how much memory fitted. Depending on the make of BIOs this quick test may or may not be displayed on the BIOS startup screen.
Second, interrupt the BIOS POST routine, by pressing the Del key or F2 or F10 depending on which BIOS you have. Once into the BIOS you should be able to see how much memory is installed.
In Windows go to Settings -> Control Panel -> System. The processor and memory details should be displayed here.
Anohter thing you can do is visit
Memory upgrades, flash media, and usb storage at Crucial.com and run their system scan. It will install an ActiveX component and scan your system, identify the motherboard and tell you how much memory fitted, how many memory slots availabale and how many used and what possible combinations of memory you can get to have the maximum fitted.
Try those and get back to us.