If the ball joints are really bad, you can see movement with the weight on the wheels. I recomend looking under the front while someone rocks the steering wheel. You can see alot. Here is a list.

The steering box could be loose. (on the frame)
The sector shaft could be loose. (bad steering box)
The pitman arm could be loose. (on the splines)
The drag link to pitman could be shot.
The drag link to right wheel could be shot. (tierod end)
The tierod end at the left wheel could be shot.
The tierod to draglink could be shot.
The balljoints can be loose.
The axle pivot mounts could be loose.
The wheel bearings can be loose.

In addition, you can look down from the top and see if the pitman arm moves imediately when the steering shaft is twisted. (play in the box)

All of these can be seen if you make sure you know what you are looking at and what it is moveing in relation to. One thing, the wheel bearings will not move (even if their shot) when the brakes are on or hanging up.

I assume you are worried about the balljoints because it is one of the more difficult things to repair on this list. If you are that worried, jack it up and do as Greystreak says. It will be much, much easier to see movement that way.