My friend got hers tested via dexa. She came out to 18%. She is pretty lean. You can make out arm definition on her and can obviously see definition. Prior to this I would have guessed that she was 12%.
That would be very hard to do. Most people don't keep records of other people at different body fat percentages. That is why general statements are made. At 10%, MOST people should be able to see their ab muscles.
Above is a picture of Bob Sapp:
In that pic he is probably around 10-11% body fat. That is just an educated guess based on taking a good number of percentages over a few years as a PT. Chances are, he isn't below 10% and he may even be closer to 12%.
At around 8%, you are fairly lean but no striations are usually visible until you get closer to 5-6% which is what most bodybuilders compete at, especially on the NPC level. That level usually isn't easy for most to maintain and still make much physical progress.
On the other end, 15% is usually about the point at which all of your abs have taken a hike and you are looking pretty filled out. 18-20% would cause most smaller people to pass for "chubby" and more muscular people to get described as "stocky".
Everyone carries weight differently, however, as I had one frat brother who taped out at over 16% body fat, but every ab was visible and there is no way anyone would have guessed him at more than 10%. That is why the number means so little. It doesn't matter if you tape out at 10% body fat...if you have no muscle and it just doesn't look good. The number means shit for most people. They are better off going by the mirror.
You do realize that bodyfat is different for women. For one thing, they have those nice little things we call boobs. 12% for a woman is pretty damn lean. Even many hard-training women with smokin bodies will never see it (unless they're training for a competition in which case they may briefly dip into the single digits. which is scary!).
Plenty of food and plenty of rest. Oh, and the occasional "cheat" meal. I keep my workouts REAL short: 5-10 steps at a time, some cell-phone curls, table pull-ins, alternating single arm ass wipes. I make sure I keep my workouts under 2 minutes per day.
Don't go by calipers too much. I had a three-site caliper test and a underwater test taken on the same day.The calipers put me at around 7% and the underwater put me at 13.6% How accurate those numbers were though was unclear b/c: 1) I had consumed a lot of alcohol the night before 2) I was on 1-ad and thus holding an additional 5-10lbs of "water weight."