First off, we need to know the context of “false news.” There are two types of incorrect informations: misinformation and disinformation. Misinformation comes from a root that is not aware of the false information that they are spreading. They genuinely think it’s true. Misinformations can be hard to spot, because media informations are often educated, meaning that the misinformation may very well be disguised as the truth. You need to examine the information further, gather relevant evidence to see if it supports the information, check the date, and consult with an expert.
Disinformations are malicious false informations that are intended to be fake. This can often be a propaganda to manipulate public perceptions on a subject. You can check for disinformations by examining the sources because political medias often spreads fake news. Biases are also common in disinformations so look out for critical opinions from a given media.
Satires are also false news. Although it may not be vicious, it still presents itself as a false information. Check if it relates to humour or any popular modern references.
You can prevent false informations from spreading by condemning it in any ways, but only after you verify that it is false informations. You should correct others once you identify that they are under the impression of fake news. For misinformations, call out the information itself and try to correct it. For disinformations, call out the information AND the source of the information to prevent it from exerting more disinformations. Satires are usually harmless, but apply all rules above if you feel that it is irresponsible in the public perceptions.
Hope this helps!