How to determine that the visitor cuts ads?

Question

Internet advertising

There are messages like "Your ads are blocked" - how is it determined, in JS or PHP? What mechanism underlies the definition?

Answer 1

This is done in JS. I assume that by the phrase "cuts ads" you mean - removes or hides it from the page. If so, then with the help of JS we can check whether the blocks with ads are removed (or hidden) or not.
Like this:

if(    $( '#blockID' ).length == 0 
    || $( '#blockID' ).css( "display" ) == 'none' 
    || parseFloat($( '#blockID' ).css( "opacity" )) != 1.0)
{
    // Реклама удалена
}

Answer 2

This is not explicitly defined in PHP. I think it's possible to check the existence of an ad unit on a page using JS. (search for, say, the selector $('#ads') )