Radiation can be detected using a cloud chamber, which contain supersaturated air-alcohol vapor. When radiation particles travel through the cloud chamber, they produce ions which in turn produce condensation, forming visibly cloud trails. Alpha particles, being highly ionising and heavy, produce short and thick tracks. Beta particles, being less ionising and lighter, produce long thin tracks. Gamma particles are usually too weakly ionising to be detected.
The following video uses a GM counter to demonstrate the range and penetration ability of alpha, beta and gamma radiation.