Modern Vespa

Is there any way / trick to check the thickness of the rear brakepads without removing the rear wheel?

Use a borescope

I can't remember back to my MP3 - but apart from that, on every other Piaggio I've owned it's possible to easily see the end tabs of the pads where the retaining pin goes through. Just by eye you can see the gap between each side, and the thickness of the rotor (disc). Half the difference, and that's the thickness of the pads. Takes just a few seconds - and if it looks like they might be fairly worn, just ensure to change them at the next opportunity.