Check the chephalothorax, Atrax robustus have a rather flat cephalothorax whereas it is strongly raised in Hadronyche sp.
Atrax robustus (a bit beat up I know):
There are off cource other differences as well in the spinnerets or the tribal spurs, you can also look at the filling of teeth in the fang groove. But that is hard if not impossible to do on a living specimen.
hmm do you think i have a chance to tell the difference between atrax.r and any other spider in the world when i look at the flat cephalothorax and the shorter spinnerets? or are there many other with maybe the same flat c. and short s.?
its not really a question of live and death but i just wanna be sure ;-)