Muon identification is possible because muons are the only charged particles which can penetrate large amounts of material. As described in spec:calorimetry, electrons and hadrons interact destructively in material. Muons, due to their large mass relative to the electron, do not suffer from such catastrophic interactions. The method for detecting muons, then, is to place charged particle detectors behind a large amount of shielding material (typically steel).
The FOCUS experiment has two muon detection systems. The Inner Muon detectors use common scintillator detector elements. The Outer Muon system uses resistive plate chambers to detect passing muons.