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So how does the fisherman know that the “wild” fish they caught was raised in a hatchery?

They don't necessarily know. The whole point of releasing months-old salmon fry and smolts into the wild to mature on a wild diet- as opposed to raising them to marketable size in a fish farm- is that they're indistinguishable from truly wild fish (they put on >99% of their body weight on wild food sources) from a consumer standpoint. Most of the fish my association releases weigh about as much as a penny at release. The rest of their adult body mass comes from foraging in the wild.

Coho and Chinook are generally tagged by the hatcheries prior to release, but not at a 100% rate. Tagged fish usually get an adipose fin clip, so they are visually distinguishable. Pinks, chum, and sockeye are generally 100% otolith marked- temperatures are manipulated when they're still in the incubators to lay down dark and light bands in their otoliths (ear bones). Those fish can only be identified by cutting the otoliths out of their spine after harvest and examining them under a microscope.

The marking is especially important because the salmon hatcheries up here are actually run by private non-profits (PNPs), not the state. In addition to direct cost-recovery harvests, our funding comes from an ex-vessel tax on the fisherman that's based on how many hatchery-origin fish they caught, thus a lot of fish get sampled at the processing plants to determine their origin. It's a very different system than the state-run hatcheries prevalent in the lower 48. Basically the commercial fishermen banded together to create the PNP hatchery associations in the 1970s after over-harvest of wild fish almost destroyed their livelihoods. So now we supplement wild runs with hatchery fish, which both allows more sustained harvest and helps protect the wild runs from being wiped out. And it's funded by the fishermen themselves.