Wren nest: Too high up, too late?

We put up a standard bird box earlier this year and we now appear to have wrens in there. I can see that there is a nest in there and it's clearly a wren that has been going back & forth for the past month or two but I know the male builds several nests and the female chooses only one. So how do I tell if there are eggs/young in there without disturbing them? The nest is on a north facing wall of the house right underneath our (1st floor) bathroom window so quite high up for wrens. Now we are seeing much more activity  - one or two adults fetching food & taking it into the nest but I can't hear any noise from the nest box. Is that likely if there are babies in there?