It might be more useful to tell us
why you want to know this as it's not quite as straightforward as you might think - definitely not as straightforward as simply measuring the peak signal level on a spectrum analyser with an antenna.
Answers range from making a crude 1/4 wave ground plane antenna (for example), rectifying it and sticking it into a moving coil meter, to spending a million quid getting a field strength and reflection plot of your entire house at different polarisations etc.
None of this is useful if you are trying to do something in the real world as your device probably has an F antenna printed on a PCB, buried in the device somewhere as generally people prefer shiny glass or metallic-looking plastic to F antennas poking out the side. In which case, for real world results, use a PCB type F antenna, but then you must bear in mind that this doesn't represent absolute field strength.
Please elaborate.