The plugin checks for you, once a week. If a destination has stopped answering — usually because somebody deleted or renamed the page it pointed at — the redirect is marked in the list, counted on the Dashboard, and you can filter the list down to Needing attention. There is a Check them now button too.
This matters more than it sounds. A redirect pointing at a dead end catches a visitor and then drops them anyway, which is no better than the dead end it was meant to fix — and it is the one problem you would never think to look for, because the rule is right there in the list looking fine.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
