In Windows, click on the Start button, select Control Panel and open System. This will display basic information about the computer including the amount of RAM installed.
In MacOS, select "About This Mac" or "About This Computer" from the Apple menu in the upper left corner of your Desktop. This will provide information about your Mac's total memory.
In Linux, open a terminal window and enter the following command: cat /proc/meminfo
This will show the total amount of RAM as well as other memory information.