There's a pretty good article on Wikipedia about a few good methods for determining body fat: http://en.wikipedia.org/wiki/Body_fat_percentage. A couple of these can be done on your own, such as water displacement and skinfold caliper measurements, others need sophisticated equipment one will only find in medical facilities or high end gyms.
WHY generic BMI charts fail: they don't account for muscle mass and bone structure. I can tell you now from a lifetime fitness class I took I score several points better on these charts than my actual height-weight reading will give. (I can guarantee I will never see the 180 or lower that it recommends for a 6'2" man. If I do, I'll be in the hospital for cancer or something dying.)