There are two easy and fast methods. The first is to right click on the MyComputer icon found on the desktop which will also show the user name, make and model cpu, and version of Windows running on the system along with the rounded off total amount of memory. The second and more detailed is seen during the initial post tests when the system is first turned on. With the logo screen disabled the total amount of installed memory detected is counted and displayed.
When the memory is loaded with a number of startups you will notice a slowdown due to not having enough ram available for running programs. To determine if the memory is working properly there are a few stress testers like memtest available. Everest and other benchmark programs will also show the total on memory along with other system information.