WordPress at a glance
function is not described

upload_is_user_over_quota() WP 3.0.0

Check whether a site has used its allotted upload space.

No Hooks.

Return

true/false. True if user is over upload space quota, otherwise false.

Usage

upload_is_user_over_quota( $echo );
$echo(true/false)
If $echo is set and the quota is exceeded, a warning message is echoed.
Default: true

Code of upload is user over quota: wp-admin/includes/ms.php VER 5.0.3

<?php
function upload_is_user_over_quota( $echo = true ) {
	if ( get_site_option( 'upload_space_check_disabled' ) )
		return false;

	$space_allowed = get_space_allowed();
	if ( ! is_numeric( $space_allowed ) ) {
		$space_allowed = 10; // Default space allowed is 10 MB
	}
	$space_used = get_space_used();

	if ( ( $space_allowed - $space_used ) < 0 ) {
		if ( $echo )
			_e( 'Sorry, you have used your space allocation. Please delete some files to upload more files.' );
		return true;
	} else {
		return false;
	}
}