How can the signal strength of an RF signal be measured?
Andy is correct - we really need to know the details about frequency, power out etc. to give a precise answer but as a general solution you will require a pick-up/tuned circuit (LC), a sensitive detector (diode) and some sort of buffer/amplifier (possibly a FET/mosfet/op amp) and a meter to indicate the field strength. 

Accurate calibration of what the meter reading actually means is of course a different matter but if you just want to detect a particular field strength for proximity you could always use a comparator to set the point at which the field strength will exceed a certain value of voltage output.