If you didn't want to rely on the signal whistle... you could still visually check the pressure with the manometer.
Not all the machines had a manometer, it could be bought separately.
The sufficient pressure to use the coffee machine was 1 to 1,5 ATÜ.
ATÜ stands for "Atmosphere Überdruck".
1 ATÜ is equivalent to 0,981 bar.
If you still have the original manometer on your machines, the right pressure was signaled by a red line, like on these machines i have seen so far: