What should they have known?
Rashi: Who made the foundations of the land - they should serve Him!
Radak: You should not err to serve idolatry. You should know yourselves that Hashem is the master. If you do not know yourselves, you should know from what you read in Seforim! If not, you have a tradition from the beginning! And you should have understood from the foundations of the world! The idol worshippers are not so foolish to think that the wood or gold created the world, and it is the master. Rather, the idol is made for the sake of a star or upper form. They thought that serving the idol is like serving the star. Later generations believed that serving the image can help or harm, like it did for our fathers. Nevi'im rebuked them - "Af Lahem v'Lo Yerichun" (Tehilim 115:6). They thought that the images can help or harm; they did not put to heart that a Creator made the world, and benefits the good and harms the evil. This is Darchei ha'Emori!
What should they understand about Mosedos ha'Aretz?
Radak: Everything is from the four 'elements' (earth, fire, wind and water), and combinations of them. The elements have different natures. Surely a conductor set limits where they will be, lest they confront each other, and established their natures, which never change! Also, elements of opposing nature co-exist b'Shalom in living beings. The world is like a house that Hashem built. He appointed man to oversee it. Man did not see the Master, but he knows that before him, his father oversaw the house. If he is intelligent, he will seek the Master to ensure that he oversees it properly! A fool does as he desires; Hashem will uproot him from the world. Refer to 40:23:1:1.
Malbim: It is one of the four ways they should know. (a) It is an axiom - "ha'Lo Sed'u." (b) It is famous - "ha'Lo Sishma'u." (c) It is a tradition - "ha'Lo Hugad me'Rosh Lachem." (d) From the world's foundations, the simple elements and their compositions, you can see and understand Kel's strength and ability.
