White space, case, curly and straight quotes, dashes and a quote's own quotation marks and final punctuation do not count; any other difference does (a changed word, a paraphrase, an invented sentence). Deterministic, and costs nothing: a check for a judge written as an ordinary AI function that gives a score and the quotes it rests on.
Examples
source <- "The parcel left Leeds on Monday. It was delayed by snow."
quotes_found(source, c("\u201cIt was delayed by snow\u201d", "It was lost"))
#> [1] TRUE FALSE