Simply running the Ratsnest command will tell you how many airwires there is left on the board or, if the board is fully routed, it'll say Nothing to do!. It still doesn't help you find WHERE the airwires are though.
Exactly.
To find the last, very short, nearly unvisible, airwire, do as follows:
Zoom out till your board is just one or a couple of square cm's on your screen.
Hit the ROUTE command.
Now click in the middle of your board. The new trace will automatically snap to one end of the airwire.
Now zoom out.
You have found the airwire.