There are some inspection points that will allow you to easily verify the depth of your bore holes:

The pipe will be delivered / brought to the job site already cut to match the depth of the bore holes (see first image below).  Look for some kind of tag / paper work indicating the pipe is the correct diameter / length.  Illlustrated here is 1" HDPE U-Tube pipe at 300' per roll (600' down + back).

After each hole is drilled, you can observe the pipe being inserted into the hole (see second image),

and verify that all of the pipe is inserted (third image).  Here it shows what was left hanging out of the ground for each 300' spool.

You can also verify the holes are being drilled to the proper depth by counting the number of 10' drill rod segments on the drilling rig.  There were 30 on the rig in the second image.

Hope this helps.

Best regards,

Bill