To do that, one needs to go back to get_post_format()
as the Codex page explains:
$format = get_post_format();
if ( false === $format )
$format="standard";
So in order to check if a post is in standard format, I can use this shortcut:
if ( false == get_post_format() ) {
// I'm a standard format post
}
/* or */
if ( ! get_post_format() ) {
// I'm a standard format post
}
Or reverse the scheme to check if a post is just not in standard format:
if ( false !== get_post_format() ) {
// I'm everything but a standard format post
}
/* or */
if ( get_post_format() ) {
// I'm everything but a standard format post
}