The phrase "I know it when I see it" is a colloquial expression by which a speaker attempts to categorize an observable fact or event, although the category is subjective or lacks clearly defined parameters. The phrase was famously used by United States Supreme Court Justice Potter Stewart to describe his threshold test for obscenity in Jacobellis v. Ohio (1964). Obscenity is not protected speech under the Miller test, and can therefore be censored.

I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"]; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that. [Emphasis added.]
—Justice Potter Stewart, concurring opinion in Jacobellis v. Ohio 378 U.S. 184 (1964), regarding possible obscenity in The Lovers.

The expression became one of the most famous phrases in the entire history of the Supreme Court.[1]

Stewart's "I know it when I see it" standard was praised as an example of candor[2] and "realistic and gallant."[3]

A well-known quotation of this phrase is found in Goldfinger, when M asks Bond: "What do you know about gold?" and Bond replies with "I know it when I see it."[4]

C&P from Wickopedia