Pull out the calipers and measure the fork. Then measure the ID of the race. You gotta have numbers. There's 27.0 and 26.4mm. You very well may have a 27.0mm race on a 26.4mm fork. The high-end headsets are pretty much all 26.4mm and the el-cheapo dept. store bikes have 27.0. Someone might have just installed a cheapo headset in your bike at one point.