how to tet RF delay

Does somebody has some good idea about RF system test?
The delay is from DAC through two stage mixer to RF 2.1GHz ouput signal.

You can use a low frequency modulation and compare the phase difference between your input signal and the envelope of the output signal. Start with a low frequency signal to make sure you are measuring less than a cycle of phase and slowly raise the frequency to get better accuracy in your phase measurement.

This only works if the time delay is long enough. Also try a pulse input signal with a longer off time.