$WOOCS->get_checkout_page_id()

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This function will help you to define checkout page. Use it to define that current page is checkout.

global $WOOCS;
if(is_page($WOOCS->get_checkout_page_id())){
    //do smth here if current page is checkout page
}

global $WOOCS;

if(is_page($WOOCS->get_checkout_page_id())){

//do smth here if current page is checkout page

}

Better use in your site header.php OR in wp_head hook callback