a Velodyne SMS-1 will measure it for you, and let you dial your sub in too.
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Any one of several CDs with test tones and an SPL meter will let you do it manually. The Stereophile Test CD #2 http://ssl.blueearth.net/primedia/home.php?cat=2 and the analog Radio Shack SPL meter is what I've used in the past.
The Velodyne SMS will only measure up to 200hz or so, which is great for setting up a sub, but it is very important to measure the whole system's response for a full picture.
The test tone CDs + SPL meters will only give you a few points on the chart, which is certainly better than nothing, but a very rough estimate at best.
Your best bet is to get a calibrated microphone (+ mic preamp) and use it, in conjunction with software like TrueRTA or roomEQwizard. With this approach, you can measure the whole system's response, and be able to answer important questions that both other approaches will not be able to.