Generally, you're only likely to have problems on your US-bound flight (e.g. SAA's flights from JNB to IAD and JFK and perhaps the Delta metal flight too).

Immediately before the actual boarding gate (which is usually at the far end of a terminal, somewhat isolated from most foot traffic at JNB), they add extra "illusion of safety" security checks right at the gate. These security checks are in place to satisfy some US "rule" crafted no-doubt by someone who has never been to South Africa. Purportedly, the security folks at this check check for, inter alia, liquids of all types. They also frisk you in a laughable attempt at discovering anything on your person and rummage around your carry on bags. I guess the idea is that, in the opinion of the good old USA, while every other international flight out of JNB is flying to a less-security conscious country, the US is the US and airlines must comply with the rules of security theatre, so the carrier (I suppose) pays for this extra level of security (and adds it to the ticket price).

I've had all sorts of things confiscated for no reason whatsoever (try explaining the US TSA intricacies to these folks or SAA staff and you'll get nothing but a blank stare). Items confiscated have included and been varied as unlabeled liquids, liquids that we "too big" (actual quote "too big" before removal from my oft-carried US small ziplock - other larger volume items left in the bag) and so forth.

But, enough of my rant. My advice is to have a "small" labelled (e.g. commercially purchased) bottle of any liquid/gel/cream you need for your flight to put in your carry on for the flight home from JNB to the USA. I'd go with the small plastic ziplock bag concept too as that seems to placate the folks somewhat and you can take it out and make it obvious for them (that seems to make it more legit that you're showing it to them rather than trying to sneak it through).

Anything larger (e.g. wine, your $50 bottle of shampoo/conditioner, cologne, etc.) or likely to cause problems (e.g. unlabelled bottles) should be checked. The forum is rife with stories one way or the other about luggage pilferage at JNB, review those threads before deciding whether it is worth it to put anything of value (liquid or otherwise) in your checked bags.

My experience is that with intra-South Africa domestic flights, they could really care less what you're carrying on. That's a joke. But, in CPT domestic and JNB domestic security (and a long time ago in PLZ and, I think, DUR), I've received quizzical looks when asking if I should remove laptops, electronics, shoes, etc. before passing bags through the XRay or myself through the scanner. I've routinely set off the scanner machine and given a "oh it must be my cell phone" look and they just wave me by. I've carried raw Impala filets, wine, whisky, etc. through domestic security at JNB and CPT for a domestic intra-SA flight without issue. And, on several occasions, have rolled my baby in a stroller through the scanner with a diaper bag on back (the metal of the stroller obviously setting off the machine) and the security officers barely look up from cooing over the baby.

Can't speak for JNB to Vic Falls routes.