Jack the front end up, or put the whole bike on a stand. With no weight on the front end, you will be in a much better position to see where the play is....
If I had to guess, I would say it's the a-arm bushings. The wheel bearings are pressed into the hubs with a spacer inbetween the two and do not get their adjustment from the castle nut. If the bearings have side to side play, then it's the bearing itself that is worn out.
With a stock arm setup, the "plumbness" is not adjustable, toe-in is adjusted by the tie rod.