Admittedly, it
takes a bit of courage and "thick-faced" when doing all these bus
related stuff.
The easiest and surest way to know the gear box is to look at the
driver's dashboard. Most of the MB0405s, Volvos have the
gear-change at the dashboard, and look out for 3 buttons "DNR"
arranged horizontally. Besides these buttons there is usually
either a circle with ZF inside to say the bus is ZF-ed, and if
there is a word Voith in caps, then it is Voith-ed. However, most
of SBST MB0405s do not have these symbols because of they have MB
gearboxes.
It takes a bit of craze to actually trace the gaer changes when
relaxing or studying on the bus, and it sort of becoming natural
after some time. For me, to differentiate the gearbox, I normally
listen to the first gear change and the retareder.
If the first gear change is sharp and goes like
"piiiiooooooooouuur", then it is definitely ZF
If it is muted, then it is Voith or MB, and they can be
differentiated by their retarder
If there is a nice whistling sound as the bus decelerates, it is
Voith. If there are 3 stages of "whoooooo..." then it is MB for
SBST0405s
If there is no distinct sound but you can hear the gear "dropping"
back to neutral, then it is ZF
The endangered LO2-Ax have Allisons.
Some of the SBST DM 0405s have Voiths instead of MBs
Hope to be of help 
