How do I trigger Appstore check for app updates manually? I'm using a firewall and by default, everything, including checking for updates is blocked. 

In some situations, I'd like to disable the firewall and trigger such checks manually. How do I do that?
I have the firewall turned on on my computer and I don't have a problem checking for updates.  I go to the App Store and at the top of the window is an icon for updates. Click on that and it tells you the updates available.

See the image below.