Listen to a note, then find it by ear. Any octave counts — you are matching the note, not the key.
No piano? Play with your computer keyboard:
Left hand — A S D F G white keys (C–G), W E T black keys.
Right hand — H J K L ; white keys (C–G), U I P black keys.
↑ / ↓ shift both hands an octave — current offset 0.