Assuming I don't speak the languages (a safe assumption).
Carthage is directly on the sea, for one thing, whereas Rome is several miles inland in a hilly, swampy area along the Tiber. A visitor to both cities would notice the differences almost immediately, geographically they're quite distinct. Or do you mean architectural differences and whatnot?
Well, I suppose the obvious one is that Carthage was built on the sea while Rome was about twenty kilometers inland, so the scent of the sea would clue you in pretty quickly. There are also some simple differences in landscape: Carthage was built snaking alongside the hills that face the sea, the better to catch a breeze (also: it's hot in Carthage).
But I'm assuming you are looking for something a bit more interesting, so one big difference is in the sources for water. Carthage was completely without aqueducts until the emperor Hadrian constructed one so was dependent on cisterns. Roman Carthage had by far the largest cisterns in the Roman world, but they are still quite common in Carthaginian Carthage, found in many different apartment blocks.
Lamps are another one. Roman lamps look like this, Carthaginian lamps look like this.
I think it would be easier if he just looked at the harbour, he would notice two pretty large Drydocks called Cothons, don't think Rome in the 4th-3rd century had many impressive easily recognizable buildings. Anyway here are some images and the source.
https://s-media-cache-ak0.pinimg.com/736x/fb/91/6b/fb916b3f6f6bbd4582b522f5683d9927.jpg
http://www.ancient.eu/carthage/
Pg. 85, second column
