Quick! Which of these two moons was closer to the Cassini spacecraft when it took this image?
Hard to tell, isn't it? Actually, it's impossible to tell without knowing what's what.
The gray moon to the upper right is Dione, and the blindingly white moon to the lower left is Enceladus (famed for its geysers of water erupting from its south pole). When I look at this picture, I can mentally swap the two moons in distance, making one seem farther away, then closer. It's a bit like the Necker cube illusion.
But here's a big hint: Dione is Saturn's third largest moon at 1123 km (698 miles) across. Enceladus is comparatively small at 504 km (313 miles) in diameter. Since Dione is not twice the size of Enceladus in the picture, it must be farther away. In fact, Cassini was 510,000 km (317,000 miles) from Enceladus when it took this picture, but Dione was a more distant 830,000 km (516,000) -- more than twice as far as our own Moon is from Earth.
Note that Dione is much duller and dimmer than Enceladus; its surface is rocky while that of Enceladus is highly reflective water ice. That's why the smaller moon's image is washed out; the exposure was set for darker objects. Both moons look slightly non-circular, but that's because the Sun was very slightly off to the side when this picture was taken, so neither moon is quite "full".
Engineers and scientists know precisely where Cassini, Saturn, and its moons are at any given time by first knowing celestial mechanics: the math and physics of orbital motion. But they also get telemetry from Cassini that gives them its position. Physics is all well and good, but nothing beats a measurement in the field. And that's how they know how distant those moons were when the shot was taken. Without that knowledge, these pictures would still be amazing and important to planetary astronomy, but would hold a lot less scientific value.
As usual, you have to be careful when looking at pictures of objects in space. Your brain is accustomed to all sorts of cues for judging size and distance like sunlight angle, amount of detail seen, the amount of haze in the air... all of which are totally absent in space. You might think you're not easily fooled, but your brain has other ideas.
Image credit: NASA/JPL/Space Science Institute