How does Linux determine which network interface to use, when both are connected to the same network?

Note that this isn't a question on routing. I'm familiar with how that works. This is if, say, I have my laptop connected to my wireless router through both my ethernet card and my wireless card, or if I have two ethernet cards both connected to the same router.

I can say from experience that in my case, my laptop seems to favor the ethernet card (eth0) over the wireless (eth1--I know that's not a typical name for a wireless interface, but that's what I have), but I was wondering, how does it decide that? If it just picks from the lowest numbered interface, what if the two choices are, say, eth0 and wlan0?