When the electric field at the surface of a conductor exceeds the dielectric strength of the surrounding air, that air ionises in a thin layer and the corona effect appears. It never becomes an arc: it bridges nothing, but it consumes energy continuously.
It shows itself in several ways at once, and each one gives a detection method. It emits audible noise and ultrasound, which a directional detector locates; it emits ultraviolet radiation, which a corona camera sees; and it produces ozone and nitrogen oxides, which attack polymeric insulators and fittings. It also generates radio interference.
It appears wherever the field concentrates: points, badly finished fittings, damaged conductors, contaminated or wet insulators. It is a classic symptom of poor assembly or of ageing, and on extra-high-voltage lines it also represents an appreciable energy loss, especially in rain.