Go to someone with a termlab and have them check it. You will want to do all your sound deadening and other major modifications for SPL finished before you get it checked. But when you have the resonant frequency of an exterior enclosure(vehicle) you pretty much just put a sub(usually a 12") in a sealed box inside the vehicle at different locations all while running frequency sweeps and graph the results. The sweeps are usually just 20 hz to 80hz, and peaks in SPL, at certain frequencies denotes the peak resonant frequency.
But as mentioned before, certain factors will change the resonant frequency drastically, such as but not limited to, air temp, humidity, fat chick in front seat, and even something as small as the visors being down. This is why most people will just run a box with an aeroport because the ability to change the port length and tuning is easier. Hope this helps!