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.