You will find a section that looks like that:
ASPI: open_driver 101 HostASPIStatus=0x1 HACount=0x1
ASPI: open_driver HostASPIStatus=0x1 HACount=0x1
ASPI: S=1 ha=1, ID=7, M='ASPI for WIN32 ESDI_506', Id='ESDI_506'
ASPI: 0:0:0 'LITEON ' 'DVD-ROM LTD122 ' 'IL5L' [CDROM]
support_scsi = 1 support_ioctl = 2
ASPI: opening 0:0:0
ASPI: Error in scgo_send: s.SRB_Status is 0x4
ASPI: Mapped to: error 1 errno: 5
unit 0: Drive empty
LITEON DVD-ROM LTD122 IL5L = uaescsi.device:0
The highligted part shows the unit number.
If you see error messages or if WinUAE crashes with an illegal operation error, check and correct your ASPI setup.
ASPI is an abstraction layer which allows to send SCSI commands to both, SCSI and ATAPI (IDE) devices. Download ASPI from Adaptec: Direct linkt to the download page (do only install this on Win98 or Win95 !)
Also read the Readme.txt and UAEHowTo.txt files in WinUAE's Docs directory.
On WindowsXP or 2000 you might want to disable the Use ASPI SCSI layer option. Then the log looks slightly different and the unit number might be different, too, but you don't need to care about ASPI.