Clamp on RF current meter.
Use a split ferrite core. Wind 10 turns around one half of the core. (Each pass through the hole when the core is assembled counts as one
turn.) Connect a 50 ohm resistor across the winding, otherwise the core will choke the current, affecting the measurement. Clamp the core
around the coax you're measuring.
The actual current on the outside of the coax shield = N * V / R, where
   V = the RF voltage across the resistor
   N = the number of turns on the core
   R = the value of the resistance
For R = 50 and N = 10, the wire current in amperes = the resistor voltage in volts / 5.
To convert the RF voltage into a DC voltage, use a diode detector consisting of a series diode and bypass capacitor of around 0.001 - 0.1 uF or so.
 A (germanium diode, like the 1N34A) will allow accurate measurements to the lowest voltage.
(without the resistor in the circuit and replacing the meter with a high impedance earphone it becomes a very broad-band receiver)
Use shottky diodes, then silicon diodes for higher power applications. At moderate signal levels, you'll get the peak voltage, which is about 1.4
times the RMS value.
Use insulated telephone wire for winding. The insulation will give you the spacing between turns to minimize the eddy current coupling between turns.
Keep R1,D1,C1 at the winding if you wish to remote the meter.

If you want to know what type of core you have here is a so so way. If you have an antenna analyzer that reads R and jX, it's pretty straightforward.
Loop a single (insulated) wire through the center of the ferrite and connect it to the antenna analyzer
Adjust frequency until R=X.
Mix 33~10MHz
Mix 43~20-25MHz
Mix 61~50-70MHz
or so.

More exact info can be had by inspecting charts of R and X vs. frequency from any ferrite manufacturer, since those quantities are typically
measured for one turn (one wire pass through the center). Fair-rite Corp. has very extensive technical information with their online catalog.