JohnDallas saidUnder water weighing is the most accurate. However as an alternative I agree with the above post. With one exception the more sites that are tested the larger the margin for error therefore I recommend 4 site testing. There is less margin for error and it is also the most commonly used.
ive never heard of 4-site. 3 and 7, yes. 4, not so much.
and only do 7 if you're under at least 15% otherwise its a waste of time. i get 3 done when im getting checked. try to eliminate all other variables. use the same guy using the same machine at the same time with the same circumstances surrounding that moment of the day. you shouldn't do anything metabolic for 3 hours prior. that includes eating, coffee, walking, running, working out. basically. the distance from house to car. car to gym door's is the most you should be moving. its like getting your RMR checked, the closer you do it to the moment you wake up, the better.
and yea, thats your best bet. dunk tank is optimal, air displacement is optimal, but they're also not usually available at your gym.
above post about calipers is correct. make sure they're pulling up, not pushing in to pull up. they should usually do it and hang on to the skin for longer than a millisecond. sometimes, the best people will do three analysis jsut to make sure, or three grasps of the skin to make sure.