With the following code, you are able to detect when a page is loaded, for Windows. For the other platforms, more investigation is needed.
function TForm62.ReadyState: string;
var
pDoc: IHTMLDocument2;
ie: IWebBrowser2;
begin
Result := '';
ie := IWebBrowser2(TMSFMXWebBrowser1.GetWebBrowserInstance);
if Assigned(ie) and Assigned(ie.Document) then
begin
pDoc := ie.Document as IHtmlDocument2;
Result := pDoc.readyState;
end;
end;