How to find the SQL Server Instance name

First, check that the SQL Server service is installed in the machine/server bu using "services.msc".

For that, press WindowsKey+R to get the Run dialog. Then type "Services.msc". Then find the service "SQL Server ...".

If you get the SQL Server service in the service list, that means SQL Server is installed in this server/PC/Machine.

Now let's find the instance name...

First Option [will work for all - Recomendade]
go to start menu and find "SQL Server Configuration Manager". open it.
point to the "SQL Server Services".
from the right pane find the service "SQL Server (instance name)" [as you get in the service.msc list]
now Right click on it and go to "Properties".
now click on "Server" Tab.
you will get all the information here .. look at the below image.

Configuration Manager

Second Option [sqlcmd]
Open "Command Prompt" [ press "WindowsKey+R", then type "cmd" on run dialog]
in the command prompt console -> type "sqlcmd -L"
you will get the server name and the instance name most luckily
check below image ...

sqlcmd -L