Its hard to decribe the differences, however, when youve seen both snakes in the wild, youll immediately recognise them.
As Cameron said, the head shape is a big give away. The strongly trapeziodal (elapid) shape of the head, much like a tiger snake I suppose, lets you know its a rough scale. The large bulbous areas that create the 'trapezoid' are where the venom sacks are located. Keelbacks have solid teeth and no venom. So no venom sack bulges. The oval shaped head of a keelback is very noticably different, once youre aware.
Like bushman said also, Elapids all look pretty mean due to the supraocular scale above the eye. Keelback eyes have a much rounder appearance because of the higher and less angled supraocular scale.
The grin helps also as rough scales tend to have a straight jaw line that doesnt curve up towards where the ear opening would be.
From the side of the head, in profile, the Keelbacks face after the eye tapers to a point at the tip of the snout, whereas the Rough scale's head is much thicker and squarer and ends noticably more bluntly.
Because of the wider head, rough scales have a much more defined neck when compared to a keelback.
And of course, as bushman said, if you can get close enough, the absence of a loreal scale will determine its a rough scale. A rough scale's preocular scale, the scale in front of the eye, touches the nasal scale directly. A keelback's preocular scale and presubocular (the scale below) are separated from the nasal scale by this loreal scale.