You can use the –list-sdks command of dotnet ro check which .net core version is installed on yoru machine:
dotnet --list-sdks
The command will list the .net core version installed on your machine
6.0.301 [/usr/local/share/dotnet/sdk]
6.0.302 [/usr/local/share/dotnet/sdk]
6.0.400 [/usr/local/share/dotnet/sdk]
6.0.401 [/usr/local/share/dotnet/sdk]