How To Check Manually for App Updates

Some days ago in the bell right above in the main window NC said, there would be a problem with connecting the update server. So I have taken a look at www.mydomain.tld/index.php/settings/apps. There it said left above in the window, all apps would by up to date. Additionally, I have checked the logfile at https://www.mydomain.tld/index.php/settings/admin/logging, where I found IMHO no suspicious entries. But I am suspicious. Because for at least a week if not more, no updates for apps have been announced in the bell. That takes me wonder. So I would like to check if the update mechanism really works.

How can I do that?