On Windows, right-click the desktop and select Display settings
On Windows, look under Display resolution
On macOS, click the Apple menu and select System Settings
On macOS, go to Displays and check the resolution
On Linux, open Display settings or Screen settings
On a browser, use JavaScript: screen.width and screen.height
On a browser, use window.screen to get the screen size
On a mobile device, check the device display settings
On a monitor, check the native resolution in the manual or product specs
On a graphics settings panel, look for current display resolution
On a command line, use system tools or display queries for the active resolution