Hi!
Unfortunately, we don’t store that info when a sync is performed through the Dashboard, only WP-CLI. See the wp elasticpress get-last-cli-index command here.
You can create some custom code storing that info using the ep_after_dashboard_index action though.
As we don’t provide support here, I’m marking the topic as resolved now but feel free to open an issue in our GitHub repository suggesting the feature you’d like to see. Thanks!