Having read the answers to the How much mayhem could I cause as a fish? of the salmon mayhem gang series, I was wondering, what means the salmon gang had to actually figure the impact of their evil doings?
They are fish, I would say they can watch the ports: the military port, the Marina and the commercial port (this might not be the technical word).
If, after doing some bad deed, they see a sudden rise in military port activity and/or a changement in commercial port/Marina activity, then they know that something is up. And, since something is up, they might conclude that their actions had some repercussions (but it can be linked to something else entirely and the fish probably won't be able to know).
A more reliable solution would be for the fish to have access to the news (paper, TV, internet). This way, they can monitor if people are talking about them. But that might be more complicated to achieve even if it's not impossible.
If there is a house near the sea and the TV happen to be visible from the ocean, then the fish might be able to watch the TV and, with some luck, see there misdeed in the news.
They can also try to read the newspaper. The ocean is heavily polluted and having a newspaper through into the ocean from time to time isn't impossible. The paper will degrade quickly but the fish might get enough time to read at least part of it (and see if people are talking about them or not).
Another (less realistic and more difficult) solution
Internet cables are running through the bottom of the ocean. With a lot of technology and ingeniosity, the fish might be able to spy all internet communication and, ultimately, even connect some "fishy device" to the internet (which will make them able to "surf on the web").
Note that, since the ocean is heavily polluted, raw materials for what they need to build their device might be easily found.
Much of the expected signs depend on the plans these fishes have hatched. For instance, if they are planning a nuclear disaster, they would need to look for signs of radiation to confirm it. But here are some general indicators of chaos they can look for.
Monitor major sea-ports for reduction in activity
Reduction in commercial/tourist activity at sea-ports would be a good indicator of unusual (war-like) conditions in the world. The fishes could look for signs such as ships staying in docks for longer, lesser ships leaving the ports, or lesser ships arriving.
Look for reduction in Oceanic Pollution
Well if there is a significant reduction in human population, you can be sure that all those factories pumping garbage into ocean will eventually have to shut down. This will either happen due to lack of demand for goods or through reduction in available manpower to keep them running. Either way, there should be reduction in the stuff dumped into the oceans.
Plus all those tourists throwing plastic on beaches should reduce too. Look for these signs, and you should be able to get a reasonable idea of your progress.
Wreckage of war-ships
If you are causing an all-out war, you can expect some ships to be sunk and some oil to be spilled. It might be difficult to find the sunk ships in the ocean, but hopefully oil spills should be easier. Plus you can try some fish-crowd-sourcing to gather intel.
Or, they could just turn into humans (as I have explained here) and watch their plans unfold with their own eyes.