I'm trying to display comment meta under each comment, but I couldn't find which action should I hook in.
You will need
get_comment_meta to pull that information from the database. You will need to know the metakey. If you don't have that, you will have to search the theme/plugin that generates the metadata for the occurence of
add_comment_meta to find it.
Where to hook depends on how your theme is listing the comments. If it simply uses
wp_list_comments there is no filter to simply attach the metadata. You will need to change the callback argument using the
wp_list_comments_args filter. Then you can make a callback function that specifies which comment and meta data to display and how (learn more).