Usually the one that's causing a problem will be labelled as not responding.
The command would still works when the app is in full-screen mode. This can be used at any time but, given that you can quit any app by making it active and pressing ⌘+Q, the only time you'll need it is when an app stops responding or starts beachballing. Press ⌘+Option+Esc and you'll bring up the Force Quit dialog box.
So here's how to do Ctrl Alt Delete on a Mac:
And guess what, there's a shortcut for that as well. The simplest answer to 'How do you control alt delete on a Mac keyboard?' would be to use Force Quit.
