The only way I know is by checking child window status at frequent intervals but this is explained clearly in this question
That said, I don't know if that could be an alternative, but using a lightbox instead of a new window popup would allow you to keep full control on your events as the whole thing stays in the same window.
Most lightbox API's offer that kind of functionality (loading an external site in the lightbox instead of the usual image), using dynamically generated iFrame to display the external site. This solution also have drawbacks (e.g.: frame-busting code on site loaded in lightbox) but can look nicer than a plain old new window...
I've been using Shadowbox on projects for quite some time now and always liked it, but there are plenty of others out there, maybe even better.