You'll see a "Fact Check" tag in your Google search results telling you which claim is being checked, who made it, who did the fact check and a short summary of the fact check, like "mostly true" or "false."
And on Google News, articles that have been checked for facts will be labelled "Fact Check."
In practice, the tag will only show up on some stories — those that have been fact-checked by organizations such as PolitiFact and Snopes. Furthermore, Google says that on some search pages you might get conflicting results, as different fact-checking sites may have judged the same page differently.
If you're a publisher and want to be included in the program, there's a couple of rules you must adhere to; plus, Google will only include publishers that are "algorithmically determined to be an authoritative source of information".
Mashable has more details.