Yes, there is a way to know your version, CDDZ -- but better is an air test. If you are only air testing a quarter to 6", something is wrong -- either the machine, the coil, or one other issue could be EMI or some strange ground condition.
Do you have a place where you have little EMI (preferably not indoors) that you could test your machine? If so, here's what I would do -- set your sens. to max, and air test a quarter -- first in disc. mode with zero disc, then in disc mode with 40 disc, and then in all metal mode (threshold as high as you can run it, with just a mild, not overwhelming hum). Sweep it quickly across the coil, and then more slowly -- to find the best speed. Lay a yardstick down, to get accurate measurements. Then record all those results. If you can't do better than 6-7" on a quarter, something is wrong.
On a Gold Bug, you check the version of software as follows -- turn on the machine, put it in disc mode, and then turn it off. While off, hold down the ground grab/pinpoint button, and turn it on, then release the button. On the Gold Bug, the screen will say "Gold" then "Bug" and then "X", where X is a number -- the software version number. I don't think, though, on the G2, the software version tells you much, as I understand it. They didn't have any "prototype" G2 machines; they weren't release, as I understand it, until the "final" version of the Gold Bug, the Pro, was released. Thus, you should have up-to-date software (at least in terms of any "silver issues" that may have existed with earlier prototype GB SE machines.)
I am no engineer, but your air test that I described above should arm you with some information which will determine if, or if not, to give First Texas a call. I can tell you for a fact that even inside my high EMI house, I air tested a quarter to 11 or 11 1/2" in disc mode, and can hit it at the same depth in the ground (even deeper in all-metals mode; this is on my Gold Bug DP with 11" DD coil -- should be an identical machine to your G2).
These are really impressive, well-performing machines. I'd hate to think you have a unit with a substantial problem, and think "that's the best these things can do..."
Steve