Windows: Open Task Manager, go to Performance, then Memory
Windows: Press Win + R, type dxdiag, and check the Memory line
Windows: Open Settings, go to System, then About, and check Installed RAM
Mac: Click the Apple menu, then About This Mac, and check Memory
Linux: Open Terminal and run free -h
Linux: Open Terminal and run cat /proc/meminfo
BIOS/UEFI: Restart the computer and check the system information screen
Mobile: Open Settings, then About phone/device, and look for RAM or Memory
Mobile: Use a device info app if RAM is not shown in settings