Being able to identify a fish relies on several factors, such as the body size, shape, and pattern, certain characteristics, where it's been found, and various others. There are many tools online to help identify fish.
The first step in identifying a fish is to determine whether it is a freshwater fish or a saltwater fish, which is typically the type of water the fish is found in. From there, analyze its snout shape, tail shape, body shape, any patterns on its body and fins, and the shape of its fins and where they are located. Any distinctive features must be taken into consideration, be they something like the fish having a sucking disk instead of a jaw, whiskers or more than one spine. Even after taking note of all of these things, it takes research and refining of any key identifiers to come to the correct and final conclusion about the type of fish.
Other than doing your own research to identify a fish, there are a few easier ways. Many local wildlife websites have charts that help to identify local wildlife, including any fish that may be found in the area. A quick discussion with a park ranger or even a pet store worker can shed some light on what kind of fish has been found.