My dishwasher Fagor fi-80 just fills continuously, till overflowing. How should it know its full? I cant see any sensors.

It uses an air bell to turn the depth of water into air pressure, then a pressure switch senses this pressure.

Down inside the wash tank, you'll see the air bell. Usually grey, a flattened rounded rectangle tapering wider towards its lower end, stuck to the side of the tank.
Take the panel off the machine, and on the dry side of the tank you'll see a little hose leading away from the bell towards the pressure switch.

Four of the more common failures....
1) Water valve stuck open. It'll keep filling even with the electricity turned off.
2) Air bell leaking. Crumbling plastic bits not unknown, this machine could be 15 years old by now.
3) Pressure switch stuck. Blowing into the switch you should hear a 'click'.
4) Most common - a build up of fat in the top of the air bell, blocking the sensing tube. With the machine empty of water, take the pipe off at the pressure switch end, and blow into it. This should dislodge any fat/debris. In extremis, slacken the nut holding the air bell to the tank, rotate the bell upright and give it a good wash out.