Why would you test this? Well, because BFP is a far more meaningful measure of overall health than bodyweight alone. Most people experience an increase in their lean mass if they exercise regularly, and the increase can potentially be significant if they strength train regularly. So there can be little change in the scale, even as people are burning fat.
The link someone has posted above with the photos at various BF percentages in a great visual, but I think we all have a little difficulty in judging ourselves against these photos.
Online BF calculators are free and easy to measure, although they are not all that accurate in an overall sense. But they do make a good way to track your progress over time.
One compromise might be to use an online calculator on a regular basis, then 'calibrate' it against a more accurate method every 3-6 months.
The skinfold test with calipers is a pretty good compromise between accuracy, cost and ease of testing. Most gyms offer this for $10-20.
For an online calculator, try the 'tape' tab at www.linear-software.com/online.html