The "official" way to measure is the width of the shoulders at the acromium, which is the hard bony corner at the widest point at the corner of the shoulders.

http://www.infovisual.info/03/025_en.html

The other poster's suggestion to hold the bars probably makes more sense if you can do it.

Also remember that various manufacturers measure their bars at different places. The most common difference is whether they measure from center to center of the bars, or from the outside edges. Therefore, one brand's "44" outside-outside would be the same width as another's "42" center to center.

I can't pre-test bars so I had my wife measure my shoulders. 41 cm. My current bars are 42 c-c, so I decided to try 40 c-c bars this time. Frankly, I doubt most of us would notice a size either way, but I read that one source of shoulder /upper back pain (which I occasionally have if I don't concentrate on relaxing) could be too-wide bars, so what the heck.