Measure the peak of FFT and its frequency

Im trying to find the frequency Offset between 2 USRPs then transmit my
signal with compensating the Offset. From the forum I found out I can do
this by transmit a cosinus signal and measure the peak and its

I could manually use file sink and put it into Matlab. But i want to
build a “semi-automatic” System, that measure the Offset ,feed it into
UHD Block, then send my own Signal.

Is there some way I can do this within GRC(very unlikely i think) or