So you designed and built a box and think you know what it is tuned to, but there are a lot of reasons to verify that you are right. Some of these are:
1) To make sure you understand the math behind designing the box
2) To check up on the person that did the design for you
3) To check on your building skills
4) To verify where you are so that when you redesign to deal with an acoustic difficulty that you found you are able to.

Whatever the reason, there is a simple way to do this.

You will need the following equipment:
1) Your system installed in your vehicle.
2) A CD with a variety of low frequency tones on it, I’d use 1 or 2Hz spacing from somewhere below where you think it is tuned to somewhere above.
3) The DMM you used to set your gains (an Oscope or other voltage measuring device is also fine).
4) A resistor. Generically speaking almost any resistor will be fine, but I would highly recommend using one that will dissipate a lot of power (100w is logical) and is at least double the DCR of your coils. As a good starting point a 20ohm resistor would be fine. For example you could use this one from Mouser which is nearly noob proof for the reasons I just stated:
http://www.mouser.com/search/ProductDetail...71-HL100-06Z-20

Once you have assembled your gear, the testing is pretty easy. Here are the steps:
1) Wire your resistor in series with your driver. For example, take the + output from your amplifier and run it to the resistor, connect the other end of the resistor to the + input on your sub. Leave the – output of your amp connected to the – input on your sub. Your resistor is now functioning as a voltage divider and will let us measure a manageable voltage range on the DMM.
2) Connect your DMM across the resistor. One lead on each side of the resistor.
3) Play your first tone on your CD. Since you know the approximate tuning value of your box, start slightly lower and work your way up. NOTE: Please do not play your system loud through the resistor as it is completely unnecessary. A very moderate to low level is just fine. I will repeat: a moderate level is fine do not do this with your system wanging.
4) Record the voltage readings on your DMM for each frequency. You will end up with a table of values like I list here (note for a ported enclosure the lowest voltage value will be at the port tuning frequency):
1.4V at 28Hz
1.5V at 29Hz
1.4V at 30Hz
1.3V at 31Hz
1.2V at 32Hz
.9V at 33Hz
1.1V at 34Hz
1.1V at 35Hz
1.3V at 36Hz
In this case the ported box is tuned to 33Hz. (You do not need to use 1Hz spacing, but can use whatever you determine is accurate enough.)

For those of you with access to an arbitrary function generator and data acquisition system the process is even easier. Since I do, I set up a 0.25Hz frequency sweep from 20hz to 60hz and record the voltage across the resistor at all frequencies. This makes the whole measurement time a couple of seconds and gives a very accurate measurement for the tuning frequency of your box. The analyzer will create a table like I made by hand above and all you have to do is look for the lowest voltage.

You can do this for a Sealed box as well. When looking to find the Fb of a sealed enclosure, you can do exactly the same test only now look for the peak voltage instead of the lowest reading. In the case above if the box were sealed it would be tuned to 29Hz.

I hope some of you find this useful.