code/Debug/Model/Observer.php
public function canCaptureBlock($block)
    {
        $blockClass = get_class($block);

        if (!$this->canCaptureCoreBlocks() && strpos($blockClass, 'Mage_') === 0) {
            return false;
        }

        // Don't capture debug blocks
        if (strpos($blockClass, 'Sheep_Debug_Block') > 0) {
            return false;
        }

        return true;
    }
public function canCaptureBlock($block)
    {
        $blockClass = get_class($block);

        if (!$this->canCaptureCoreBlocks() && strpos($blockClass, 'Mage_') === 0) {
            return false;
        }

        // Don't capture debug blocks
        if (strpos($blockClass, 'Sheep_Debug_Block') > 0) {
            return false;
        }

        return true;
    }
Logic that checks if we should capture specified block

@param $block Mage_Core_Block_Abstract
@return bool