Hi,
This measurement envyronmet called: GDO...
(GridDipOscillator, & as hes name tels us, it is a very old, so ca. 80-90 years, technique)
What you need:
a sweeped signal generator (in your checked fequencies) with a relative small, but wideband coil as antenna, than if you are with them over your TX/RX(exactly a tuned-swinging coil) you has a dip (smaller signal) on your "antenna coil"/generator output, what you must know to detect.
Often works metal detectors on the same princip.
As you know; today are most/much RFID devices with same (called ASK) modulations in work too...
K.