Establishing that an ATV or UTV was defective is done with the machine itself and expert analysis. Engineering experts examine the vehicle's stability and rollover propensity, its rollover protection and restraints, and its brakes, steering, and other components, comparing them to what a reasonably safe design and manufacture would provide.
The manufacturer's own records matter too — design and testing history, any recalls, and prior complaints or similar incidents can show a known problem. A machine that was unstable, lacked adequate protection, had a faulty part, or was sold without adequate warnings can support a product-liability claim against its maker.
The catch is that the machine is the key evidence, and it can be repaired, altered, or discarded after a crash. Preserving it — exactly as it was — for expert examination is one of the most important early steps, and it is why acting quickly matters so much in these cases.