This sounds a lot like what happened to me this afternoon. Hopefully this is the right place to post this ...
I have an old (many times repaired) Eterna espresso machine. I've recently replaced the manual fill switch and the pressostat and the machine has been working great.

I had an issue today though. The HX started filling and wouldn't stop so I shut off the water flow and soaked up the water (the HX had filled and water was released through the safety valve before I had a chance to stop it).
At first I thought a sensor had blown but after a few tests, I now think that the problem is the autofill solenoid. To test this, I turned off the machine, drained as much water out of the system as I could and then opened up the water supply. Immediately, the system started to fill but did not stop at it's usual level. I tried turning the machine on and hitting the manual fill button (hitting feels good sometimes), I tried adjusting the pressostat settings but neither made any difference at all. As long as the water feed remained connected (and open) the machine would fill and not stop.
In the past, I don't ever recall the machine filling until it was turned on (and the solenoid activated). The default position for the solenoid should be off - well, I think it should, it would seem a bit odd if the default was open (or "on"). A power failure would result in a flood.
So ... my questions:
1. Is there a way I can be sure that the problem is the solenoid?
2. Are my assumptions flawed? Could the problem be something else and, if so, what is the best way for me to test?
Any help is appreciated.