Detecting should be the same as you already have. As far as I can tell, there are not online "installers" you can bundle in your application, but there are some pretty straightforward online install options.

With internet access, you can either have your installer run a winget command, or embed the dotnet-install powershell script into your installer and have it run that. Both of those should achieve an online install without packaging a bulky installer.

These options are explained and the script download is available at https://dotnet.microsoft.com/en-us/download/dotnet/6.0 for net 6.