I'd do a test at a location where you have room. Install thin walled metal conduit pipe at different distances from a planned auger cast pile. The purpose of these would be to see if they deflect or are damaged as the drill for the pile as it goes by and while withdrawing. Or use a pipe type that can break when deflected. I had a series of pvc pipes installed in a "fence" in the path of a drilled tunnel where there was a chlorine line buried a little farther ahead. Purpose was to verify that the tunnel was low enough to avoid hitting the chlorine pipe. It worked well.
Next topic: How to see what is happening. It depends on what you are concerned with. The least I would do is have a few different sized dowels, maybe a few feet long and lower them on a measuring tape and judge what is going on accordingly. This can get very fancy if you wish.
