If running Windows then go Start > Control Panal > System and it will tell you in Mb. Alternatively open Task manager (Ctrl, Alt, Del) and click on the Performance Tab. Look at the figure for Total physical memory (again listed in Mb)
If non-Windows reset the computer and during the boot sequence you should see a memory text. The figure is shown in bytes so divide by 1024 to get Kb and again by 1024 to get Mb.