How did Abel know what to offer?

His parents, Adam and Eve, tried using plants to cover themselves when they sinned. God corrected them by making coverings of animal skins which required something to die. Later, Cain offers what his parents did, plants; but Abel offers what God did, a living sacrifice, proving that he was righteous; or, living his life according to God’s standard.

References

⁴ By faith Abel offered to God a better sacrifice than Cain, through which he was attested to be righteous, God testifying about his gifts, and through faith, though he is dead, he still speaks.

Hebrews 11:4 NASB

Genesis 3:7 Adam and Eve trying to use plants

Genesis 3:21 clothing made of skins

Genesis 4:3 Cain’s offering

Genesis 4:4 Abel’s offering

Matthew 23:35 Righteous Abel

1 John 3:12 Abel’s deeds were righteous

Hebrews 12:24 Jesus’ blood better than Abel’s