If you have a Thunderbolt and have it configured for an external oscillator,  Lady Heathers'  "&a" autotune command will determine and display the oscillator EFC sensitivity.  It works by setting the DAC voltage a few millivolts high,  measuring the frequency,  setting the EFC a few millivolts low,  measuring the frequency,  and calculating the gain value in Hz/volt.  The process takes a couple of minutes.  Typical values for the standard Tbolt oscillator are around -3.5 Hz/volt.