Hi Dmitry,
In trying that approach:
bool isEdit = HttpContext.Current.Kentico().PageBuilder().EditMode;
I get...
The page builder feature is not enabled. You need to call the 'UsePageBuilder()' method of the Kentico.Web.Mvc.ApplicationBuilder instance at the start of your application's life cycle
page builder is enabled in Application_Start so I'm not sure what the issue is.