There's 2 dominating schools of thought on this one.
1 - Buy basic calipers, for about 10 bucks, and measure 3 sites on your body. You should be able to find more detailed instructions online somewhere.
2 - Who really cares what the number is? If anyone asks you, just respond with "Just under 25 percent." They probably won't know if that's high or low.
Look in the mirror while wearing a bathing suit. If you don't like what you see (as in, you wish you had abs, but don't see them), then your BF% is too high. If you're happy with what you see, then you're just right.