Well for "realism" using ears is actually not recommended because in real life it's usually not a pleasant frequency. If you're experiencing a bad resonance that might just be realism. But much like Cliff said in his notes, to get an accurate measurement you would need to measure it from your cabinet. Cabinet resonance is something else which in most cases is something that "I believe to be a bad thing" and good cabinets are intentionally designed so that they resonate inbetween notes so you're not getting a huge volume boost on specific notes. F.ex. a Mesa 4x12 have all the resonant frequencies for all dimensions so that they never hit any note in 440hz tuning. I don't know if this is intentional or not but because it's Mesa I think it might be intentional.
What you can do is enter your cab's dimensions here: http://www.mh-audio.nl/standingwaveinbox/calculator.asp and then look at those frequencies on this chart http://pages.mtu.edu/~suits/notefreqs.html and you'll see if there are some resonances that mess things up for you. Remember to enter the inside dimensions of the enclosure.
