On the taskbar at the bottom of the screen you will see an icon with a green arrow if the server is started
On the taskbar at the bottom of the screen you will see an icon with a red square if the server is stopped
If the icon has only a white circle and neither an arrow or a square, it means that the SQL Server service is not started. This could be because the user logged on to the computer does not have sufficient authority to start the service, or that the service was not automatically started when the computer was started.
If there is no icon visible at all, then SQL Server or is not installed on the computer. Sometimes if you have just installed SQL Server, the icon does not appear until you reboot the computer or start the service manually, so check this first by rebooting or starting the service before trying to install SQL Server.