So how can we tell that the Russian meteor isn't related to asteroid 2012 DA14?

One way is to look at meteor showers -- the Orionids all have similar orbits to their parent comet, Halley. Similarly, the Geminids all move in orbits that closely resemble the asteroid 3200 Phaethon, which produced them. So if the Russian meteor was a fragment of 2014 DA14, it would have an orbit very similar to that of the asteroid.

It does not...

The bright blue line in the diagram above shows the orbit of the Russian meteor prior to the meteor breaking apart over the city of Chelyabinsk. The meteor hit the atmosphere at a speed of 18 km/s (11.2 miles per second or 40,300 mph).

It was moving at a shallow entry angle (less than 20 degrees) and broke apart some 15-25 km above the Russian city. Most of the damage was caused by the shock wave produced when the meteor disrupted.

Several thousand meteors enter Earth's atmosphere each day. The vast majority of these, however, occur over the oceans and uninhabited regions, and a good many are masked by daylight.

Those that occur at night also are rarely noticed by people. Due to the combination of all of these factors, only a handful of witnessed meteorite falls occur each year. The Russia meteor was one of those rare instances.

If you look at the image, the orbit of the Earth is the green circle. That of 2012 DA14 is the blue ellipse that is almost entirely within the orbit of the Earth; notice that it is close to circular.

The other blue ellipse, stretching way beyond the orbit of Mars, is the first determination of the orbit of the Russian meteor. Notice that the two are nothing alike; in fact, they aren't even close.

This is one reason -- a big one -- why NASA says the asteroid 2012 DA14 are not connected.