Torsion bars are long metal springs that are used in low-cost car suspension. As more load is placed on a torsion bar, the more the bar twists. The amount of the twist will differ depending on the material used in the bar. Torsion bar suspension is used because it is cheap and durable, but it can still go bad. Most often torsion bars are damaged through impacts on the undercarriage or through rust.
Shine a flashlight into the wheel well of the car or truck. The torsion bar will be sticking out from the car's frame perpendicularly. Depending on the car, the other end of the torsion bar will be bolted to a control arm or the steering knuckle itself. Look for any cracks in the torsion bar. If there are any cracks in the bar, then it is only a matter of time before it cracks in half.
Feel along the torsion bar for rust. Rust patches will feel gritty like a scab. If rust is present, then the torsion bar must be replaced. Also feel the bolts that secure each end for rust. If the bolts are rusted, then it is guaranteed that the rust has spread to the torsion bar bolt holes. Make sure to repeat this process in each wheel well.
Turn the car of truck on and take it for a test drive. If one corner seems to sag excessively, then that torsion bar has gone bad. Any metal object can succumb to metal fatigue over time, especially if under constant, heavy loads. Pay special attention when driving over speed bumps. If you hear a bang, then the torsion bar has grown too soft and allowed some suspension parts to come into contact. Any worn out torsion bar must be replaced.