A simple method to find the center of gravity of a body is by the use of a plumb line.

Plumb line:

A plumb line consists of a small metal bob (lead or glass) supported by a string. When the bob is suspended freely by the string, it rests along the vertical direction due to its weight acting vertically downward. In this state, the center of gravity of the bob is exactly below its point of suspension.

Experiment:

Take an irregular piece of cardboard. Make holes A, B, and C near its edge. Fix a nail on a wall. Support the cardboard on the nail through one of the holes (let it be A), so that the cardboard can swing freely about A. The cardboard will come to rest with its center of gravity just vertically below the nail. A vertical line from A can be located using a plumb line hung from the nail. Mark the line on the cardboard behind the plumb line.

Repeat it by supporting the cardboard from hole B. The line from B will intersect at a point G. Similarly, draw another line from the whole C. Note that this line also passes through G. it will be found that all the vertical lines from holes A, B, and C have a common point G. This common point G is the center of gravity of the cardboard.