X-ray? Sure they do this often in commercial buildings but it's probably well outside of something that is cost effective in a house.
A metal detector would locate the rebar and a tone generator could be used on a wire in the conduit in order to locate it.
Since there is still some uncertainty here, I took a minute to create an image that might fix some of this.
Lightning is a DC shot of electricity going to the ground, it's source. So it's easiest to consider it that way. Next, lightning is so powerful that it creates a surrounding zone of energy on any conductive material that is able, the yellow circles. Everything that is within a distance becomes energized to a point, but is again still from the lightning and is trying to go one way - to the ground. This is why everything metal is bonded or grounded: so that nothing needs to arc to find a path, since it has it's own. Regardless of how it gets to ground, it's going there.
Back to the electrodes though. Consider my image above as being of a main structure and a garage. For all purposes necessary with lightning, there is no need to tie them together. As mentioned above about 100,000A of power coming off of a lightning strike, more rods and such can help dissipate the power faster and catch a little more of it than is going through metal in your home, but regardless it's still 100,000A of power ... what happens, happens.
Whether it's your garage or your neighbors house that is splitting the distance of the strike zone, tying them together won't fully relieve the problem of getting struck by lightning.
For purposes of the code, which you fully understand, the connection that is made between the two of them is your EGC that is only for ground faults. The reason this is in the code is because some people think that running a ground rod will be the miracle solution to ground faults and that the wire isn't necessary. This isn't the case though.
For ground faults, the EGC is the answer because it sends the fault back to it's own source - the utility. For small voltage differences throughout a system, a ground rod (or multiple for higher systems) is driven. However, for lightning, you are again giving the ground rod as a path for the voltage difference caused by it, but tying the full system together doesn't help all that much more than one rod.
Hopefully this answer helps a little better.
Best Answer
Basically not without chiseling (or drilling and chiseling) in to find rebar (and it would have to be a 20 foot chunk unless it was all tied properly for use as a ground even though you forgot that part at the time.) Covered by 2" concrete is not actually inaccessible, particularly if the concrete is relatively fresh/green. You can do that, and patch where you chisel out, or you can drive a few extra ground rods (one more will technically do, but they are not that expensive...)
Note if you have (code acceptably, but practically dubious) buried your rod horizontally at 3 feet, do consider actually driving it and any friends you give it in. I have 5 driven from the bottom of a 3 foot trench (so they go down to 11 feet) myself. My concrete contractor ignored local code and did not provide a ground connection as is required in all new concrete foundations here.