Just look at the front of your PC. It's just about that easy. Run the cdrecord with the scanbus flag. Look at the output. Mine looks like this:
[extraneous data snipped]
scsibus0:
0,0,0 0) 'MITSUMI ' 'CR-4802TE ' '1.3C' Removable CD-ROM
0,1,0 1) *
0,2,0 2) *
0,3,0 3) *
0,4,0 4) *
0,5,0 5) *
0,6,0 6) *
0,7,0 7) *
It looks like my cdrecord is device 0,0,0. From now on we'll refer to that as dev=0,0,0 (because that's how cdrecord likes to think of it).