Themes’ style.css files are not enqueued for printing automatically (see the developer handbook) – instead this responsibility is left to the theme developer. As such, you’ll need to delve into the parent theme’s source files to determine the appropriate handle. That said, developers sometimes default to using 'theme' or 'style' handles.
Note that the developer may have opted to load the stylesheet directly in their header file, or a function hooked to wp_print_styles/wp_head by explicitly printing an appropriate <link> element, in which case there will be no handle and you may need to replicate the corresponding function/template in your child theme or alter action priorities in order modify the loading process.
Further, if your child theme’s files prevent the parent theme’s style.css from enquing in the first place, you’ll need to manually register or enqueue the parent theme’s style.css if you wish to use it as a dependency. get_template_directory_uri() is useful for this task:
wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );