Open a command prompt and enter the version command.
This should return a version number. If you get something like the message below, .Net Core is not installed.
'dotnet' is not recognized as an internal or external command, operable program or batch file.
You can also issue the info command to get a lot more information about your environment.
If you want to actually look at the different .Net SDKs which you have installed, they can be found here on a Windows machine: