is there a piece of equipment that can locate the reinforcing steel in concrete. It really needs depth and size. We were asked to evaluate a retaining wall that was not inspected while being built
This piece of equipment is called a conquest sensor software, they retail for thousands of dollars and are not cheap to rent either. What they do is run the scanner (shown on the table) over the slab or wall and it will image on the screen with some waves if a bar or live wiring is present. They can accurately estimate the position and depth of the reinforcement but I doubt it will be able to tell you the size of the bar. If you have a very experienced person operating the equipment they may be able to inform you on what size they expect the bar to be based on the strength of the signal.
There are several manufacturers who make such equipment...Hilti as noted by dik is one (Ferroscan and PS20), James Instruments is one (R-meter or pachometer), and Proceq-SA (Profometer). We have both the PS20 and Profometer and have used the R-meter/pachometer extensively. All have their limitations.
You can also use ground penetrating radar and infrared thermography.
Engage a local testing laboratory. They should be able to do it.