And he said, Lord GOD, {b} whereby shall I know that I shall inherit it?

(b) This is a particular motion of God's Spirit, which is not lawful for all to follow, in asking signs: but was permitted for some by a peculiar motion, as to Gideon and Ezekiel.

Wesley's Notes

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, For the strengthening of his own faith. He believed, Ge 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. For the ratifying of the promise to his posterity, that they also might believe it.

Matthew Henry's Concise Commentary

15:7-11 Assurance was given to Abram of the land of Canaan for an inheritance. God never promises more than he is able to perform, as men often do. Abram did as God commanded him. He divided the beasts in the midst, according to the ceremony used in confirming covenants, Jer 34:18,19. Having prepared according to God's appointment, he set himself to wait for the sign God might give him. A watch must be kept upon our spiritual sacrifices. When vain thoughts, like these fowls, come down upon our sacrifices, we must drive them away, and seek to attend on God without distraction.