You will most likely be able to hear knocking coming from one side of the car, also if you jack the car up and try to move the wheel and see if there is any excessive play/movement.
the knocking sound could come from both sides if the arb bushes are that far worn. Also, you won't be able to detect the play of the roll bar bush by doing that - the other bushes perhaps.
The ARB bushes don't usually cause clunking (unless they have totally disintegrated), this symptom is most often down to worn drop links.
they don't have to be totally disintegrated and do clonk when they're worn.
you can check visually, take the wheel off, find the bush and look at the bar going through it - can you see any space for movement in there? grab the bar and push and pull it up and down, does it move a lot inside the bush clamp?
If the answer to either of these is yes then replace the bushes, it's rarely a difficult job and it's probably worth getting some powerflex ones for arb bushes. it won't make the ride any harder and will firm the car up a tiny bit, plus you shouldn't have to replace them again really.
if the arb bushes are fine do the wheel grabbing and moving thing (you should probably do that before taking the wheel off anyways).
B@