public bool IsPortalProject {get;}

{
  projectPath = Project.Current.Url;
  // A portal project path looks like this:
  //@"https://<ServerName>.<Domain>.com/portal/sharing/rest/content/items/1a434faebbe7424d9982f57d00223baa";
  //A local project path looks like this:
  //@"C:\Users\<UserName>\Documents\ArcGIS\Projects\MyProject\MyProject.aprx";
  bool isPortalProject = Uri.TryCreate(projectPath, UriKind.Absolute, out Uri uriResult)
                         && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);
  if (isPortalProject)
  {
    System.Diagnostics.Debug.WriteLine("This is a portal project");
  }
  else
  {
    System.Diagnostics.Debug.WriteLine("This is not a portal project");
  }
  // Use isPortalProject;
}

{
  var isPortalProject = Project.Current.IsPortalProject;
  // Use isPortalProject; 
}