Test for if you’re on 1st page of paginated post (with nextpage quicktag)

As suggested by @s_ha_dum, you can check the value of page. The value is 1 when you’re on the first page.

global $page;
if ( $page == 1 ) :
    // you're on the first page
endif;

But probably you need to check if the post is paginated first:

global $multipage;
if ( 0 !== $multipage ) :

    global $page;
    if ( $page == 1 ) :
        // you're on the first page
    endif;

endif;