Highlighting post author comment is a old and popular feature but now WP Owner Mark, a new WordPress plugin lets you display a special mark only on the comments made by website owner (i.e. the site admin). There are no options and settings, its an activate & forget plugin.
To start simply install and activate WP Owner Mark in your WordPress site and that’s all. Upon activation the plugin will add a small tick icon on all comments made by the site admin, as shown in the given screenshot.