If you've ever seen a puppy running it will sometimes move its hind feet to one side or the other to keep them from running into the front ones. Instead of running in a straight line it looks like the dog is running slightly sideways.
When a car does that it is most obvious when it is being driven in a straight line. One side will look like it is farther over than the other while it trails the front.
Another way to test it is to drive on a surface where you can see your tire tracks clearly. The front and rear track is nearly identical, so if you drive in a straight line it should look like there are only two tracks. If the car is dog-legging then you'll see the rear set of tracks off to one side of the fronts, maybe a little or maybe a lot. The important thing to do if you test it on a surface like that is to be going at a modest speed, not so slow that you can't tell if you're aimed straight, not so fast that you traverse it but might have turned the wheel slightly. Speed=greater distance in less time.
Biggest thing-keep it straight during any test.