How do you check that a case citation is real (and not AI-invented)?

Short answer. Open the opinion and read it, in a source that has nothing to do with the tool that gave you the citation. A citation that looks well formed proves nothing. AI tools can produce names, volumes and pages that lead nowhere, or that lead to somebody else's case. You are finished only when four things check out: the case exists at that address, your pinpoint lands on the passage, the passage supports your sentence, and the case is still good law.

The four checks [House style]
1. Identity. Pull the opinion from the court's website, govinfo, the reporter, or a free archive such as CourtListener. Compare the caption, court, docket number and date with the citation you were given. A search hit or another case's citation is only a lead.
2. Location. Open the page you cite, in the same version of the opinion you name, and search the opinion for any words you quote.
3. Support. Read around the passage until you know whose words they are: the majority, a concurrence, a dissent, or the court summing up a party's argument.
4. Status. Run a citator to see what later courts did with the case, then read those later decisions yourself.

Why "the volume and page exist" is not enough
A widely cited example is Mata v. Avianca, Inc., 678 F. Supp. 3d 443 (S.D.N.Y. 2023). Lawyers opposed a motion with a filing built on opinions that ChatGPT had invented, and the court imposed a $5,000 penalty on them and their firm. The court's findings show how the fakes failed. Several invented citations pointed to real volumes and pages that hold different cases. The fake opinions also cited real cases for words those cases do not contain. And when one lawyer asked ChatGPT whether a case was real, it replied that the authorities it had supplied were real and available through Westlaw and LexisNexis.
Two lessons follow. Never ask the tool that wrote a citation to vouch for it, and never treat a matching address as proof that the case is the one you named.

The rules behind it
[Requirement] In a federal civil case, the lawyer or party who presents a paper to the court certifies, after an inquiry reasonable under the circumstances, that its legal contentions are warranted by existing law or by a nonfrivolous argument for extending, modifying or reversing the law or for making new law. Fed. R. Civ. P. 11(b)(2). The Second Circuit has read that duty to mean, at the very least, that lawyers read the authorities they rely on, which is how they confirm those authorities exist and are still valid. Park v. Kim, 91 F.4th 610 (2d Cir. 2024) (per curiam).
[Jurisdiction-specific] Some courts and individual judges now have their own rules or orders for AI-assisted filings, asking filers to disclose the use, to certify that they checked the citations, or both. They change what you vouch for, not how a citation is written. We keep a dated list, with each rule's effective date where its source states one and a link to its text: https://citationcode.com/court-rules/ai-certification (opens in a new tab)

What a checker can and cannot do. Our citation checker looks case citations up in published court records (the first five in a draft on the free trial) and flags one it cannot find, or a case name that does not match the case at that address: https://citationcode.com/checker (opens in a new tab)
That covers part of the first check. It cannot tell you whether the case says what you cite it for, or whether it is still good law, so the reading is still yours.

If you find a fake
[House style] Take it out, then find real authority for the point or cut the sentence. Don't paste the fake citation into your memo as a warning to others: copies spread, and the next search may pick them up. If it is already in a paper that has been filed, raise it with your supervising attorney right away.

Read more
The Check Your Citation chapter (free to read): https://citationcode.com/book/check (opens in a new tab)