Depends. One of the common sorts of detectors is called a scintillation detector, basically it’s a big hunk of special plastic/glass with a camera attached to it. When particles decay inside the material, they release photons of specific energies. These photons are then detected by the “camera” and the intensities of the photons can be used to figure out what particles decayed and what the energy/mass of the particle was.

Another kind of detector relies on a semi conductive tube with a thin wire passing through it and an inert gas filling the space (proportional tube particle detector). The exterior tube is grounded relative to the wire, which is held at a high voltage to induce a steep electric gradient inside the tube. Decays and electrically charged particles passing through the tube will create a pulse along the wire, which can be measured.