How did the sailors find out who was to blame for the storm?

Answer: They cast lots

The sailors cast lots to learn who was responsible for the calamity, and the lot fell on Jonah.

““Come!” said the sailors to one another. “Let us cast lots to find out who is responsible for this calamity that is upon us.” So they cast lots, and the lot fell on Jonah.”— Jonah 1:7 (BSB)