Show which iPads have been synced
Look in preferences in iTunes and list all the devices that have a backup stored on the computer. If you didn't get a backup, then it becomes much less likely to catch any information from the system logs.

Grab yourself a copy of the iPhone configuration utility from the enterprise support site and it will log each serial number and details every time it's running and you connect a device.

The Apple Configurator does similar but is intended more for management than just remembering all the devices. It also only runs on Macs running 10.7.2 or later.