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_Macro
{%PortalContext.ViewMode|(equals)LiveSite#%}
You can place our own CSS by hiding that webpart on live site
{% if(Cookies.cmsviewmode == 3) {return true;} else {return false;} #%} //This is for page tab
{% if(Cookies.cmsviewmode == 6) {return true;} else {return false;} #%} //This is for design tab

Kentico 8 - bool selectOnlyPublished = PortalContext.ViewMode.IsLiveSite();

http://devnet.kentico.com/Forums/f45/fp27/t18261/Programatically-Determine-If-Running-CMS-Desk-o---.aspx
using CMS.PortalEngine;
if(CMSContext.ViewMode == ViewModeEnum.LiveSite)

if (CMSContext.ViewMode == ViewModeEnum.Preview) // Shows only Preview mode

this is complete enumeration of viewstates you can use:
namespace CMS.PortalEngine
{
    // Summary:
    //     Page mode enumeration
    public enum ViewModeEnum
    {
        // Summary:
        //     Live site mode
        LiveSite = 0,
        //
        // Summary:
        //     Preview mode
        Preview = 2,
        //
        // Summary:
        //     Edit mode - content editing (Page Tab)
        Edit = 3,
        //
        // Summary:
        //     Edit mode - content editing (editing disabled)
        EditDisabled = 4,
        //
        // Summary:
        //     Edit mode - not current page
        EditNotCurrent = 5,
        //
        // Summary:
        //     Edit mode with disabled controls - when not authorized or cannot edit (DesginTab)
        Design = 6, 
        //
        // Summary:
        //     Design mode with disabled controls - for portal engine pages
        DesignDisabled = 7,
        //
        // Summary:
        //     Edit form mode - for portal engine pages
        EditForm = 8,
        //
        // Summary:
        //     Unknown / does not matter
        Unknown = 9,
        //
        // Summary:
        //     Properties tab
        Properties = 10,
        //
        // Summary:
        //     Product tab
        Product = 11,
        //
        // Summary:
        //     User widgets mode
        UserWidgets = 12,
        //
        // Summary:
        //     User widgets mode with disabled functionality (for preview mode)
        UserWidgetsDisabled = 13,
        //
        // Summary:
        //     Group widgets mode
        GroupWidgets = 14,
    }
}