There are several ways to determine how overweight, or fat, you are.
The first, and most obvious way to determine if you are overweight, is by what you see when you look in the mirror. Another indicator you may be carrying more fat than you like is by the way your clothes are fitting. This is probably the first clue many people have that they may be gaining weight. Are your clothes tighter, or feeling more snug, than they did just a month ago? If so, that may be a good indicator that you have recently added some extra fat to your body frame.
If you're interested in a more scientific way to determine how much extra fat you may be carrying, there are several different ways that you can go about this. The first, and most accurate way, is to find a facility that does hydrostatic testing, or underwater weighing. This process involves being "dunked" while wearing a swim suit, in a large tub of water, and then weighed under water. People with more body fat will weigh less in water, and thus have a higher body fat number, wWhereas those with less body fat, and more muscle, will weigh more. This method is very accurate.
Facilities in your community that may have the proper equipment to carry out this type of body fat testing are often found at universities in the health science department. Also, some higher-end health clubs and gyms will often have the equipment needed to do such testing as well. One plus of having your body fat determined at a gym, or similar facility, is that if you do find you're not happy with the results, there will be professionals there to help you plan a program to get into better shape.
Another, more often used, way to determine body fat is by the use of calipers. Calipers are used to measure skin thickness on various parts of the body. The measurements are taken by grasping, or pinching, folds of skin, with the thickness of that area used to determine the amount of fat in the body. The caliper method is easy to do, portable, and relatively inexpensive. However, the results from the caliper method are not as reliable a determination of body fat as the hydrostatic testing.
Probably the easiest way to determine how fat you are is to use an online BMI (body mass indicator) calculator. You'll input your weight and height into the calculator; it will then give you a number that indicates your percentage of body fat. While this is probably the least reliable method of determining body fat, it will still be a good indicator of whether you're carrying excessive body fat.