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firstly let me say that I aint no expert on Fat, but I found this article on the Muscle and Fitness Hers Website and thought I'd share it with the rest of you 
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Technique: Dual Energy X-Ray Analysis
Precision:+/-<1%
Cost:$60-$100 per test
Time:~5 min.
Sources of Error: Most precise
Advantages: Fast; most accurate; clothing okay
Disadvantages: Not portable; apparatus is expensive; rare (only at hospitals, univeristies)
Technique:Underwater weighing
Precision:+/-1%
Cost:$25-$50 per test; ~$10,000 apparatus
Time:1 hour plus (may include enema for best results)
Sources of Error: Bone density (underestimates % of fat in African-Americans); high muscle mass (overestimates % fat in bodybuilders); gases in lung and gut; equation used (age-, race-, gender-, and activity-specific)
Advantages: Relatively accurate; very consistent (great for changes in % bodyfat); former gold standard; BIA and skinfold equations are based on this.
Disadvantages:Uncomfortable; wet; bulky apparatus; skimpy outfit required; time-consuming; expensive; hard to find (for example, colleges); bone density assumption causes error.
Technique: Bod Pod
Precision: +/-1%
Cost: $30-$65 per test; ~$30,000 apparatus
Time: 5-8 min.
Sources of Error: Same as underwater weighing; temperature; hydration
Advantages: Relatively accurate; very consistent (great for measuring changes in % bodyfat); comfortable; very fast; easy to use; usable by frail, elderly
Disadvantages: Bulky (but portable); skimpy outfit required; expensive; may be hard to find; bone denisty assumption causes error
Technique:Skinfold (Fatfold) Caliper
Precision: +/-3.5% to 4%
Cost:$15-$400 one-time purchase; $0-$35 per test
Time: 10-15 min. (includes repeats and calculations)
Sources of Error: Measuring technique; fat compressibility; age; equation used (age-, race-, gender-, and activity-specific)
Advantages:Cheap; convenient; fast; portable; useful for detecting changes in % bodyfat
Disadvantages: Requires precise and consistent technique; not accurate for very thin or very fat people; only measures fat under the skin; some caliper brands are unreliable; not an accurate estimate of actual % bodyfat; based on underwater weighing (same flaws)
Technique: Circumference Measures
Precision: +/-3.7% to 5%
Cost: free
Time: ~5 min.
Sources of Error: High error due to holding breath, tape measure placement, etc.
Advantages: Fast; free with tape measure
Disadvantages: Not very accurate
Technique: Waist-Hip Ratio
Precision: --
Cost: free
Time: ~5 min.
Sources of Error: High error due to holding breath, tape measure placement, etc.
Advantages: Fast; free; predictor of heart disease and diabetes risk
Disadvantages: Doesn't give information on % bodyfat
Technique: BIA
Precision: +/-3.5% to 4%
Cost: $100-$400 one-time purchase; $0-$35 per test
Time: ~1 min.
Sources of Error: Dehydration, bladder status, temperature; fat asymmetry (hand-held devices test upper body only, bathroom scales test lower body only); arm position; nearby appliances; exercise; equation used (age-, race-, gender-, and activity-specific)
Advantages: Cheap; easy to use; portable; extremely fast; accurate for estimating total body water in populations if height, weight and other variables added
Disadvantages: Poor accuracy detecting changes in % bodyfat (reading fluctuate too much); many other variables needed for accuracy (for example, weight/height); based on underwater weighing (same flaws)
Technique: Near-Infrared Reactance
Precision: >5%
Cost:data not available
Time: ~1 min.
Sources of Error: Measures only at 1 site; very unreliable
Advantages: Fastest
Disadvantages: Measuring at one site of little value (worse than skinfold methods); flawed overall
Technique: Body Mass Index (BMI)
Precision: >5%
Cost: Free (height and weight scales)
Time: ~5 min.
Sources of Error: Posture (height), hydration and bowel status (weight)
Advantages: Fast; free; predictor of life expectancy
Disadvantages: Doesn't give reliable estimate of % bodyfat for athletes
_________________
Technique: Dual Energy X-Ray Analysis
Precision:+/-<1%
Cost:$60-$100 per test
Time:~5 min.
Sources of Error: Most precise
Advantages: Fast; most accurate; clothing okay
Disadvantages: Not portable; apparatus is expensive; rare (only at hospitals, univeristies)
Technique:Underwater weighing
Precision:+/-1%
Cost:$25-$50 per test; ~$10,000 apparatus
Time:1 hour plus (may include enema for best results)
Sources of Error: Bone density (underestimates % of fat in African-Americans); high muscle mass (overestimates % fat in bodybuilders); gases in lung and gut; equation used (age-, race-, gender-, and activity-specific)
Advantages: Relatively accurate; very consistent (great for changes in % bodyfat); former gold standard; BIA and skinfold equations are based on this.
Disadvantages:Uncomfortable; wet; bulky apparatus; skimpy outfit required; time-consuming; expensive; hard to find (for example, colleges); bone density assumption causes error.
Technique: Bod Pod
Precision: +/-1%
Cost: $30-$65 per test; ~$30,000 apparatus
Time: 5-8 min.
Sources of Error: Same as underwater weighing; temperature; hydration
Advantages: Relatively accurate; very consistent (great for measuring changes in % bodyfat); comfortable; very fast; easy to use; usable by frail, elderly
Disadvantages: Bulky (but portable); skimpy outfit required; expensive; may be hard to find; bone denisty assumption causes error
Technique:Skinfold (Fatfold) Caliper
Precision: +/-3.5% to 4%
Cost:$15-$400 one-time purchase; $0-$35 per test
Time: 10-15 min. (includes repeats and calculations)
Sources of Error: Measuring technique; fat compressibility; age; equation used (age-, race-, gender-, and activity-specific)
Advantages:Cheap; convenient; fast; portable; useful for detecting changes in % bodyfat
Disadvantages: Requires precise and consistent technique; not accurate for very thin or very fat people; only measures fat under the skin; some caliper brands are unreliable; not an accurate estimate of actual % bodyfat; based on underwater weighing (same flaws)
Technique: Circumference Measures
Precision: +/-3.7% to 5%
Cost: free
Time: ~5 min.
Sources of Error: High error due to holding breath, tape measure placement, etc.
Advantages: Fast; free with tape measure
Disadvantages: Not very accurate
Technique: Waist-Hip Ratio
Precision: --
Cost: free
Time: ~5 min.
Sources of Error: High error due to holding breath, tape measure placement, etc.
Advantages: Fast; free; predictor of heart disease and diabetes risk
Disadvantages: Doesn't give information on % bodyfat
Technique: BIA
Precision: +/-3.5% to 4%
Cost: $100-$400 one-time purchase; $0-$35 per test
Time: ~1 min.
Sources of Error: Dehydration, bladder status, temperature; fat asymmetry (hand-held devices test upper body only, bathroom scales test lower body only); arm position; nearby appliances; exercise; equation used (age-, race-, gender-, and activity-specific)
Advantages: Cheap; easy to use; portable; extremely fast; accurate for estimating total body water in populations if height, weight and other variables added
Disadvantages: Poor accuracy detecting changes in % bodyfat (reading fluctuate too much); many other variables needed for accuracy (for example, weight/height); based on underwater weighing (same flaws)
Technique: Near-Infrared Reactance
Precision: >5%
Cost:data not available
Time: ~1 min.
Sources of Error: Measures only at 1 site; very unreliable
Advantages: Fastest
Disadvantages: Measuring at one site of little value (worse than skinfold methods); flawed overall
Technique: Body Mass Index (BMI)
Precision: >5%
Cost: Free (height and weight scales)
Time: ~5 min.
Sources of Error: Posture (height), hydration and bowel status (weight)
Advantages: Fast; free; predictor of life expectancy
Disadvantages: Doesn't give reliable estimate of % bodyfat for athletes
