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For 1. In principle, the refractive index of a true vacuum is identically 1. For air at atmospheric pressure, the index is 1.000293 for visible light. Therefore, you should be able to determine the deviations in refractive angles for a jar fill with air and one under vacuum. Since we're talking deviations on the order of one in ten thousandth, it's be quite difficult to measure this as a demo, but it could be done in the lab. The buoyancy/weight of the container could also be used to differentiate it from one filled with air.

For 2. A lot of metals have solid room temperature vapor pressures in the ultra-high vacuum region. For instance, if you had a chunk of Cadnium of Zinc in your container, all you'd need to do is shine a heat lamp on the container heating the metal by a few dozen degrees Celsius and they'd begin to evaporate away. These materials have vapor pressures around 10^-8 mBar somewhere between 300 to 400 K.

One more fun way to determine the success of your vacuum process would be to watch a Crookes radiometer as you lower the pressure. As you reach below 10^-6 mBar, the rotation should stop.