How do we know that non-hechshered food is not kilayim or kilay hakerem?

Question

"Kilayim" and "kilay hakerem" are two classes of prohibitions on eating from "mixed" fields (in which more than one type of crop is planted.) "Kilayim," as I understand it, specifically refers to the prohibition on wheat and barley grown together, whereas "Kilay Hakerem" refers to a prohibition on "eating from a vineyard with other seeds mixed into it."

According to the Mishna (Kidushin ch.1, #9), these laws are not actually restricted to the land of Israel, but must be observed worldwide, since they are halacha l'Moshe mi-Sinai.

So how do we know that fruit, vegetables, wheat, barley etc. that are sold commercially outside of Israel (without a hechsher) follow these laws? If not, why are we allowed to eat from them?

Answer 1

Rambam writes (Maachalos Asuros 10:11-12):

ספק ערלה וכלאי הכרם בארץ ישראל אסור. בסוריא והיא ארצות שכבש דוד מותר. כיצד היה כרם וערלה וענבים נמכרות חוצה לו. היה ירק זרוע בתוכו וירק נמכר חוצה לו. שמא ממנו הוא זה שמא מאחר. בסוריא מותר ובחוצה לארץ אפילו ראה הענבים יוצאות מכרם ערלה או ירק יוצא מן הכרם לוקח מהן. והוא שלא יראה אותו בוצר מן הערלה או לוקט הירק בידו

Doubtful orlah or k'laei hakerem in the Land of Israel is forbidden. In Suryah (which is the land that David conquered) it is permitted. What is a case of doubt? If there was a vineyard of orlah, and grapes were being sold next to the vineyard, or (for kilayim) there were vegetables being sold next to a vineyard, and one is uncertain if they came from this field or from another. In Suryah, it is permitted; outside of the Land, even if one saw the grapes coming out of an orlah field, or vegetables coming out of a vineyard, it is permitted, so long as he didn't see those things being harvested.

כרם שהוא ספק ערלה או ספק כלאים בארץ ישראל אסור ובסוריא מותר ואין צריך לומר בחוצה לארץ

A vineyard that is doubtfully orlah or k'laei hakerem is forbidden in the Land of Israel, permitted in Suryah, and obviously also permitted outside of the Land.

(free translation)