What are the visual differences between the F/A-18 Hornet/Super Hornet and the EA-18G Growler?
You asked about visual differences. The main visual difference is that there are no guns on the EA-18G, just AIM-120 AMRAAM or AGM-88 HARM missiles. Other than that the visual differences are minimal. The airframe is still a F/A-18F Super Hornet, where -F stands for twin-seat. Engines are also the same General Electric F414-GE-400, however I suspect there could be some differences in regards to the engine auxiliary equipment, such as the electrical generator. I also suspect there is additional equipment added for the sole purpose of conditioning the electrical power and insulating everything to keep interference minimal. It is even possible that there are optical fibers in areas where on the F/A-18F copper cables are still used.

Main differences:


Dedicated electronic attack equipment, e.g. what makes the aircraft an EA-18G, has been put in the place of the cannon.
The EA-18G has one extra protruding antenna on the back, along with the satcom antenna behind it.
The missile launch rails on the ends of the wings seem to be permanently modified to support the ALQ-218 Tactical Jamming Receiver, which is the same pod as on the EA-6B Prowler.


Here is an image of the EA-Growler showing some of the external modifications and what they’re for:

Source: airforce.gov.au/Archive.org