Researchers from the Australian National University have reported on the effects of a group of 20 km - 50 km asteroids that hit Earth.
ANU media release
Interesting stuff! I'm not sure how you go about identifying a meteor crater in rock 3.2Gyr old though. I'll have to keep an eye out for any papers on this to see the evidence.
Geologists are gneiss!!
I think one of the main lines of evidence - for this and any other impacts - is the Iridium Anomaly - basically a far larger amount of Iridium than average in the rocks associated with the impact. Other lines of evidence include 'shocked' quartz - a good description is here http://www.scn.org/~bh162/shocked_quartz.html and the presence of trydimite/cristobalite forms of quartz. However, many of these lines of evidence can also point at nuclear explosions (not too sure about Iridium) - was anyone testing nuclear weapons in this part of South Africa?!
Another product of an impact is a shatter cone, but I don't know anything about the levels of preservation in such old rocks on Earth.
(Photo from Ernstson Claudin Impact Structures)
Impact craters should also have veins below their cavity where breccia has been forced into cracks created by the shock wave, and in large craters a central uplift.
I believe any iridium gets spread out over a large area due to the impacting object being vaporised.