We discuss a method of probing the velocity distribution of confined energetic alpha particles resulting from deuterium-tritium fusion reactions in a magnetically contained plasma. We calculate the characteristics of the signals to be expected from injecting multi-MeV Li° into the plasma to undergo double charge-exchange reactions with the alpha particles. Neutralized alpha particles then escape from the plasma to be detected by a charge-exchange analyzer. We also examine the feasibility of producing a Li° beam of the required current and energy, and we discuss a conceptual design for an appropriate beam system.