Question

Which type of radiation would you look for in a star to demonstrate that nuclear fusion was occurring inside?

Answer 1

Neutrinos!

Explanation:

Nuclear reactions release energy through neutrinos as well as gamma rays (technically created then a positron annihilates with an electron). Unfortunately the gamma rays get re-absorbed and re-emitted many many times before they reach the 'surface' of the star. Neutrinos however can freely pass through the star from the instant they're created and thus carry with them information about the nuclear fusion happening in the stellar core.