Genesis 15:8

And he said, Lord God - אדני יהוה Adonai Yehovah, my Lord Jehovah. Adonai is the word which the Jews in reading always substitute for Jehovah, as they count it impious to pronounce this name. Adonai signifies my director, basis, supporter, prop, or stay; and scarcely a more appropriate name can be given to that God who is the framer and director of every righteous word and action; the basis or foundation on which every rational hope rests; the supporter of the souls and bodies of men, as well as of the universe in general; the prop and stay of the weak and fainting, and the buttress that shores up the building, which otherwise must necessarily fall. This word often occurs in the Hebrew Bible, and is rendered in our translation Lord; the same term by which the word Jehovah is expressed: but to distinguish between the two, and to show the reader when the original is יהוה Yehovah, and when אדני Adonai, the first is always put in capitals, Lord, the latter in plain Roman characters, Lord. For the word Jehovah see on Genesis 2:4 (note), and on Exodus 34:6 (note).

Whereby shall I know - By what sign shall I be assured, that I shall inherit this land? It appears that he expected some sign, and that on such occasions one was ordinarily given.

15:8 Whereby shall I know that I shall inherit it? — This did not proceed from distrust of God's power or promise, but he desired this, 1. For the strengthening of his own faith. He believed, Genesis 15:6, but here he prays, Lord help me against my unbelief, Now, he believed, but he desired a sign, to be treasured up against an hour of temptation. 2. For the ratifying of the promise to his posterity, that they also might believe it.