A sense of rhythm is a good sign of musical talent. Being able to sing or tap back rhythms that are heard is important on any instrument.
A sense of pitch is a good sign of musical talent. Being able to sing back pitches, recognize different pitches in a string of notes and hear chords are signs of musical ability.
The ability to sightread, playing or singing back music, is a sign of musical talent as well. This is typically what happens in an audition situation where the music on the page must be processed in five minutes or less.