I use to fly the aircraft in question and then went on to fly a Grob G115 at a different training school, that aircraft had a sight gauge to see the fuel level. There was a 30mm round hole covered with Perspex just above the flaps on the port side of the aircraft and when you looked through the hole there was a clear pipe that was running up the side of the fuel tank, and you could see the fuel level.
With regards to the fuel gauge showing full if a fault developed on the fuel gauge, I would not have flown this aircraft if the gauge showed full, you never fill the Grob fuel tank to full as it would be to heavy to take off, we use to put a max of 70Ltrs in it, but it is easy for me to make comments like this sat behind my keyboard.