What sign did Abraham's servant ask God for to identify Isaac's wife?

Answer: She'd offer to water his camels

He prayed that the girl who said, "Drink, and I will water your camels as well," would be the one chosen for Isaac.

“Now may it happen that the girl to whom I say, ‘Please let down your jar that I may drink,’ and who responds, ‘Drink, and I will water your camels as well’—let her be the one You have appointed for Your servant Isaac. By this I will know that You have shown kindness to my master.””— Genesis 24:14 (BSB)