How can I test my new cloner to see if it is working properly?

To test a newly built unit to see if the bubbling is going to do the trick, simply dry off the tubs lid and place it on the tub, turn on the air pump and let it run for about five minutes. After around five minutes have a look at the inside of the lid.

It should look like the pictured lid.
Not quite soaking wet yet, but obviously getting hit by the water droplets. Now put the lid back on and run it for another five minutes, it should be completely covered with those tiny water specks or totally wetted.

If its not "completely covered" all the way across the surface, then either your air pump is too weak, or the bubbling device is too small or needs to be repositioned in the tub.