Hey @jfrisbie, great question.
First, check this thread for help resolving the “USB driver not found” issue: https://community.spark.io/t/usb-driver-not-found/673
Have you downloaded the Windows driver linked from the docs?
http://docs.spark.io/#/connect/connecting-your-core-connect-over-usb
It will be easier by far to use USB to get the ID, however, yes, in fact, if your Core is connecting to the Cloud, you can find out the ID over Wi-Fi!
When the Core finishes handshaking with the Spark Cloud, it multicasts a little presence announcement message on the local network. If you either just listen with wireshark or tcpdump, or else make a program that listens for the specific announcement, you could find it.
We’re using the CoAP standard, so the multicast address is 224.0.1.187, port 5683.
https://github.com/spark/core-firmware/blob/master/src/spark_utilities.cpp#L369
The message is 19 bytes long, a 7-byte header followed by the 12-byte ID:
I’ll follow up soon with a little script that would catch it on the local network.