Trouble is I've never tried this myself, because this thread is the first time I've thought about it. I might actually be wrong on a few counts, but since acoustics isn't my schtick I'm allowed to be wrong
Anyhow. With one omni you can measure pressure. Take a second omni placed close to it and if you subtract one from the other you measure pressure differential ie. acceleration along the axis of both microphones. Integrate this signal and you get velocity.
Acoustic impedance is pressure over velocity, and at the resonant frequency of the bass trap, acoustic impedance found near it should be real, meaning that pressure and velocity would be in phase.
So if you measure pressure and velocity near the bass trap while some subwoofer elsewhere in the room does a frequency sweep, you should be able to see at which frequency velocity & pressure are in phase, which would be the resonance frequency of the bass trap.
At least I think this should work, I hope some of the real acoustic experts can chime in.
But I understand you're looking for something more practical. Is there anyway you can cover over, seal or otherwise reversibly incapacitate the bass trap?