Bechoros 19b: If one buys an animal from a non-Jew and does not know if it has ever given birth in the past…
Bechoros 21b: If one buys an animal from a Jew, and the Jew does not tell him whether the animal has previously given birth, and now it gives birth, is the baby a bechor? Rav said: It is certainly a bechor, because if the animal had previously given birth, the seller would have boasted about it.
Taz Yoreh Deah 316:4: If a non-Jew is selling a cow and the Jew tells him that the younger it is, the more he will pay for it, and the non-Jew says it has already had four calves, he is believed, because if it had been a cow that never gave birth, he would have said so, for his own gain.
בכורות יט ע”ב: הלוקח בהמה מן העובד כוכבים, ואינו יודע אם ביכרה אם לא ביכרה וכו’
בכורות כא ע”ב: איתמר. מצינו לוקח מעובד כוכבים, לוקח מישראל מאי? אמר רב: בכור ודאי, דאם איתא דבכרה ־ אישתבוחי הוה מישתבח ליה.
Rav Yisroel Belsky once allowed a kashrus agency to rely on non-Jewish workers in checking eggs for blood spots by offering a financial reward to the workers for every egg that they found containing a blood spot. Eventually, the innovation was discontinued. Apparently, one of the company workers was a diabetic who carried around a lancet. The incidence of blood found in eggs was significantly higher during this period.
Source: https://vinnews.com/2022/11/24/adrianna-the-non-jewish-nanny-and-halacha/
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