The best way is to get a personal trainer at your gym, who is trained to do it, to measure it using calipers.
You can purchase calipers or digital calipers over the net and have a family member (preferably) or yourself do it... but there is human error involved..u have to be on an empty stomach, untrained, same time of day.. grab the skin at the same angle in the same exact place... it's not full or fool proof unfortunately.
The other best ways are to take progress pictures once a month... the camera doesn't lie. The next best way is to measure your waist with measuring tape and track that progress.
I have no idea how accurate it is, but mybodycomp.com can give you a bodyfat% if you input a ton of measurements.
Whatever method you use, It's not so much what the % is.. but the improvement of the % that matters.