In Bereishis 37:29, "And Reuven returned to the pit and look! Yosef wasn't in the pit." But the verse or Rashi for that matter do not explain if he ever found out what the brothers had done. The narrative simply proceeds with the cover up.
Pirkei D'Rebbi Eliezer (Chapter 38) says that when Reuven returned and found the pit empty he accused the brothers of killing Yosef. They then told him that they sold him instead.