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The problem with Bioelectrical Impedance is that what you input is what you will get for an output. The person administering the test needs to know your weight, height, age, and activity level. If some of those variables are guessed or assumed, the results are not going to be accurate. Also, in a closed scientific setting, hydration and types of fluids ingested can cause for skewed readings, so there is a lot that can go wrong with this test, which can cause the standard of error to be fairly high (much as 40%).
The third method of testing percent body fat is called, Skinfold Measurements. This is probably the most common test you will encounter in fitness facilities and the big chains (Skindex) and it will be done by the use of skin fold calipers. When someone administers this type of test they will be taking skin fold measurements from as many as 3 to 7 sites across the entire body.  The way this test works is that the person who is conducting the skin fold measurement is going to pinch certain amount of skin from your body and firmly pull the fat away from the underlying muscle (does not hurt). When the person has the amount they are looking for they will take the calipers (like tongs) and perform the site measurement three times in a row to make sure they have a correct reading.
This type of method is an excellent tool to utilize in the gym setting because there are not a lot of variables to control. In addition, to make the standard of error associated with this test even smaller, the person administering the test can take more than three site measurements in order to have a good distribution of numbers to calculate the individual’s percentage of body fat. Also,  this test is a great motivational factor because when the client sees that their skin fold has gone down and their body fat percentage has decreased, they not only hear the numbers from the training professional, but they visually see the Skinfold and that in itself is motivating to keep improving.
One of the major drawbacks of using the Skinfold Measurement test is that it should be performed by the same person who administered the test to that client. When a client has gone through various trainers at a club, each person has their own method of where exactly they like to do their pinch test.  It is not beneficial to the person getting the test done to have someone different, they should pick an individual who is has been at the facility a long time and is a seasoned veteran at doing this test. If the person administering the test is not familiar with doing skin fold testing or is a new trainer, I would recommend having someone else do it that is experienced.
In the case where the Skinfold Measurement technique is not going to work well with a morbid or super morbid obese client, then I would recommend either taking body circumference measurements and/or the Bioelectrical Impedance method (if available).