On Windows, right-click the desktop and select Display settings
On Windows, look under Display resolution
On Mac, click the Apple menu and select System Settings
On Mac, go to Displays to see the resolution
On Linux, open Display settings or Screen settings
On a browser, use JavaScript: screen.width and screen.height
On a website, open developer tools and check the viewport size
On Android, go to Settings and open Display
On iPhone, go to Settings, then Display & Brightness
On a monitor, check the model specifications or on-screen menu
On a TV, open Picture or Display settings
On Windows, use the command line with wmic path Win32_VideoController get CurrentHorizontalResolution,CurrentVerticalResolution
On Mac, use the terminal command system_profiler SPDisplaysDataType
On Linux, use xrandr