Answer:

- In DS-User > Logs > Activity Log, hightlight the intended backup activity

- click on "Event Log" button

- look for an entry like:

'Reading 'disk-nr.vmdk' using transport mode 'certain-transport mode ' (vm_name)', for example:

Reading 'disk2000.vmdk' using transport mode 'hotadd' (w2k12r2-vm1)

or

Reading 'disk2000.vmdk' using transport mode 'nbdssl' (w2k12r2-vm1).