Determining peak instantaneous power in waveform

[John Ackermann N8UR]

Hi --

I have broadband HF data recorded off the air (ham radio bands 384 kHz 
wide) and am trying to figure out what sort of gain settings I need to 
play it back on a USRP without distortion.  My worry is the way multiple 
fairly strong signals could add together in phase to create momentary 
spikes way higher than the visible average (I think this is called crest 
factor in digital signals?).

Is there a (relatively) straightforward way to measure this with 
Gnuradio blocks?  If so, can anyone point me in the right direction to 
build a GRC flow graph to read an IQ file and print out the peak 
instantaneous power seen in the file, relative to 100% (ie, I don't need 
calibrated numbers, just to know how to set the gain in Gnuradio for 
undistorted playback).

Thanks!
John