Lifting the frames is the only way to be sure.
You can look through the end window, to see if the middle frames look full/capped, but that view can be deceptive too. Mine looked like this 2 weeks ago:
Those 2 middle frames looked full and capped. However, when I lifted them, they both had a huge empty arc of open cells below the capped honey. If I had drained them without inspecting, I would probably have lost a lot of honey leaking back into hive from the frame faces.
I thoroughly recommend inspecting the frames before harvesting.