public bool IsPortalProject {get;}

 // 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");
 }

var isPortalProject2 = Project.Current.IsPortalProject;