Chmfhm,
You will need a dial indicator to check backlash as well as steps per unit. You can pick up one pretty cheap at places like Harbor Freight. As Mactec54 suggested you should first check your steps/unit on the Z axis. If you can tell the Z axis to move exactly one inch does it move that far or is moving more or less. If you command a move of 1" it should move exactly 1"

Backlash is when you move one inch in one direction and command it to return to the starting point and it does not come all the way back. It is a little more involved but in layman terms this is checking for slop in the ballscrew / nut combination.