On Windows, open Settings > System > About and check Installed RAM
On Windows, press Ctrl + Shift + Esc, open Task Manager, and check Memory under Performance
On Windows, press Windows key + R, type msinfo32, and check Installed Physical Memory
On Mac, click the Apple menu > About This Mac and check Memory
On Linux, open a terminal and run free -h
On Linux, open a terminal and run cat /proc/meminfo
On Linux, open a terminal and run sudo dmidecode -t memory
On Android, go to Settings > About phone > RAM or Memory
On iPhone, check device specifications in Settings > General > About or look up the model online
On Chromebook, open Settings > About ChromeOS > Diagnostics or use the Chrome Task Manager
On any computer, check the device specifications label or manufacturer support page
On any computer, use a system information tool such as CPU-Z, Speccy, or HWInfo