Many people who have a lot of hands-on experience in a field can tell when something is ready, or a piece of equipment has a problem by sense-the sound or sometimes the smell-(bouquet), the feel, or the look of something.
Picking and pressing grapes into wine after many hours and after many years, can also be like that. After many years you can tell when grapes are ready to harvest. This same experience applied to crushing and pressing grapes. After many hours of pressing grapes we should be able to tell by the sound of the grape seeds inside the press, that there is no more free run, and it is time to start inflating to press the grapes to retrieve the remaining juice. While many have computerized presses and lab equipment these days, we like many wineries that still go by sense or experience, and confirm it up with lab tests or computer equipment. I reprint a section from The Wine Manual with respect to free run.
"Free Run is a substantial amount of the juice of the grapes, more than half, that pases through the tiny perforations in the stainless steel cage of the press. This naturally flowing juice is called the free-run. It is the sweetest, least astringent juice, considered to be of the highest quality.
Several types of presses are used in modern winemaking. Modern state-of-the-art wineries are equipped with computer-controlled presses which regulate the whole cycle. Some common presses include the basket press, the continuous press, and (the most gentle) the horizontal pneumatic membrane press.
This last type contains an airbag that runs the entire length of the press. While the must is pumped in, the airbage is collapsed. Once the press is loaded to capacity and the cage is closed, the airbag is inflated and the must is gently pressed through the perforations, and funnelled into fermentation tanks. Skins, stems, and seeds are retained inside the cage." The Wine Manual by J. Marie