I'm sure you'll get the "airlock activity is not a reliable gauge of fermentation activity" response at some point. It's not absolute, but it's certainly a reasonably safe bet imho. Is there any other activity going on in the brew? Krausen? Foaming? The faint sound of bubbles bursting as they hit the surface? A small amount of bubbles visible at the surface around the edges (much like how beer sits in a glass)?