There are many different ways to measure body fat. Unfortunately, most methods aren't practical or accurate. Generally, the more accurate it is, the less practical it is. Here are some popular ways of measuring body fat percentage.
7-site/5-site caliper testing
Whichever method you decide to use, make sure it's one you can do consistently. Just like measuring body weight, it's less about the number and more about a data point to track progress over time.