Cloud chambers!

One of the original detectors in physics was the cloud chamber. The basic idea is that you have a very pure gas with high humidity and cool it off or reduce its pressure until it's on the verge of condensing. Since you have essentially a pure gas without any impurities or dust particles, there's nothing for the water to condense on. As a result, the air in the chamber is supersaturated, and it will condense at the introduction of any small impurity. Luckily for physicists, the chamber can be tuned so that a charged particle passing through the detector is just enough to cause condensation. So a charged particle leaves a track of condensation as it passes through the gas. If you're lucky enough to have a particle decay or otherwise interact inside the detector, you can see all the (charged) tracks involved in the interaction.

Once you have that set up, you can produce a nice, uniform magnetic field throughout the detector, and then a low-momentum particle will spiral in a characteristic way, while a higher-momentum particle will curve only slightly. At this point, the early physicists took pictures of the chamber for later analysis (often by human "computers"). The resulting images are entrancing and emblematic of the early days of particle physics.

A somewhat stylized cloud chamber image, courtesy of the CERN Courier.