play by ear

1. Play a musical instrument without the aid of written music, as in By the time she was four, she could play a dozen songs by ear . [Late 1600s]
2. play it by ear .  Proceed gradually, depending on the circumstances; improvise. For example, I’m not sure how much we should say about our plans, so let’s play it by ear . [Mid-1900s]

from dictionary.com