Proving and demonstrating vacuum in container without breaking it Let there be a hollow container made of glass or some other transparent material, roughly the size and shape of an apple. Let the walls be of sufficient thickness for the container to be safely evacuated to some reasonable degree, perhaps around $10^{-8}$ mbar, and then hermetically and evenly sealed. 


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*How could one prove that the container is evacuated, and with what accuracy? Would x-ray crystallography, laser scattering, light absorption or emission or ultrasound be possible ways? 

*Is there a simple way of demonstrating that the container is evacuated, perhaps by placing something inside it before sealing which behaves in a very specific and obvious way in a vacuum, without actually removing said vacuum? Aside from the obvious feather, which would fall without air resistance. I was thinking of something along the lines of a small quantity of cesium, but that wouldn't be distinguishable from an inert gas atmosphere. 
Thanks!
Edit: There is no reference container to compare with and the container itself isn't standardized, so density/weight considerations are out, if I am right. Actually, so is light refraction, probably, as the container walls aren't really level enough, considering the small refraction delta mentioned in Andrew S.'s reply. 
 A: 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 between in refractive angles for a jar filled 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 (down scroll for the charts) 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.

A: You can use electric discharge of appropriate frequency, as its threshold in gas depends on pressure (and frequency). 
A: Freeze it in liquid helium. Any gas inside will condense out.
Spin it quickly then stop it. The internal turbulence of the spinning gas will be visible with a sensitive detector.
Apply a short sharp impact to one side. If there is gas inside, the sound energy peak from the sound transiting the gas will be temporally distinct from the spectrum of the sound transiting through the glass.
A: It all depends on what kind of sensitivity you want in pressure measurement. If you just want to distinguished a vaccum state from atmospheric pressure, then there are many non-destructuve ways:


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*index of refraction of air at atmospheric pressure (and a few orders of magnitude down) is different from 1 - optical interferometry.

*if you apply a temperature difference, you should be able to see turbulence with Schlieren photography. Measuring thermal conductivity is a worse idea because the container walls will conduct the majority of the heat.

*light absorption measurement in gasses - spectrometry - is also a very sensitive method.

*listening for the echo off the opposing wall could measure the speed of sound.


All four methods eventually reach the limit of sensitivity when the pressure is sufficiently low - below some threshold, you will just see no signal. The method #2 is qualitative (gas or vacuum, true or false). The other 3 are quantitative, they can actually measure the pressure. The first one could be problematic without calibration. #3 and #4 seem very reasonable, although I don't know how small pressures can be detected. It depends on the width of the container. #3 is probably the only one that could possibly still detect the gas content at $10^{-8}\rm mbar$. High-end gas sensors can detect trace gas concentrations in parts-per-billion (american billion: $10^{-9}$).
A: Why don't you just put a balloon inside? It will enlarge because the proportion of the pressure inside and outside the balloon will change.
