I've seen ways of seeing if a window a particular script opened is still opened, but what if it didn't?

I have a small window that has a button to click to load the large window. When I close the large one, I want a particular onUnload or onBeforeUnload to fire iff the small one is closed; if it's still open, those procedures will not fire. I may be having simply a massive brain fart but I can't figure out how to check if the other window is open. The big one isn't opening it, so I can't simply record the handle from opening it.

In shorter terms: If window A opened window B, how can I check within window B if window A still exists?