From Document 9974 of the International Civil Aviation Organization, Flight Safety and Volcanic Ash indicators that an aircraft is encountering volcanic ash are related principally to the following:
Odor: smoky or acrid odor that can smell like electrical smoke, burnt dust or sulfur.
Haze: within the aircraft cockpit and/or cabin; fine ash can settle on surfaces.
Changing engine conditions: surging, torching from the tailpipe, and flameouts can occur; engine temperatures can change unexpectedly, and a white glow can appear at the engine inlet.
Airspeed: if volcanic ash fouls the pitot tube, the indicated airspeed can decrease or fluctuate erratically.
Pressurization: cabin pressure can change, including possible loss of cabin pressurization.
Static discharges: a phenomenon similar to St. Elmo's fire or glow can occur; in these instances, blue-colored sparks can appear to flow up the outside of the windshield or a white glow can appear at the leading edges of the wings or at the front of the engine inlets.