Source

File: wp-includes/functions.php

function win_is_writable( $path ) {
	if ( '/' === $path[ strlen( $path ) - 1 ] ) {
		// If it looks like a directory, check a random file within the directory.
		return win_is_writable( $path . uniqid( mt_rand() ) . '.tmp' );
	} elseif ( is_dir( $path ) ) {
		// If it's a directory (and not a file), check a random file within the directory.
		return win_is_writable( $path . '/' . uniqid( mt_rand() ) . '.tmp' );
	}

	// Check tmp file for read/write capabilities.
	$should_delete_tmp_file = ! file_exists( $path );

	$f = @fopen( $path, 'a' );
	if ( false === $f ) {
		return false;
	}
	fclose( $f );

	if ( $should_delete_tmp_file ) {
		unlink( $path );
	}

	return true;
}