According to this knowledge base article, the registry key ARCPROWORKOFFLINE under HKEY_CURRENT_USER\Software\ESRI\ArcGISPro\Licensing provides information on whether the user has checked out a licence for offline use. However, this does not appear to be the case. I have taken a licence offline under 3.5.2 and the value is still set to FALSE.
As I am developing a script based on PSAppDeployToolkit to deploy ArcGIS Pro, I would like to check if a licence is still checked out before uninstalling. The registry key mentioned would be suitable for this if it worked properly.