James,
backlash would look like an uneven cut. The finish edge at the corners would be like a wavey line. This happens when there is play in the mechanics of the machine like a set of cogs not meshing together correctly so that when your turn one cog it moves slighlty before the other one moves. In a profiling machine that drives atart to wobble a bit at a change of direction until the machine sort of balances itself out.
t I have found that the best way the check for backlash is to run your machine at a 45 degree cut so that both x and y axis are travelling at the same time ie cut a square but rotate it to 45 degrees on the bed. The corners should be nice a smooth without any wobble in the cut surface. Cutting at 45 also tells if your speeds are the same in both axis at the same time. If they are not you would have an uneven cut surface all the way along the cut.

Ian