Sit with your student and take turns playing simple melodies with your instruments (a few keys on the piano, or a few chords strung together on a guitar). You choose the melody and play it first, and then ask your student to mimic the sound.
Take a break after each melody and ask the student if they know what notes are being played.
Depending on their performance and answers, you can choose to start from scratch with a broad overview of the instrument’s notes or build off their existing knowledge accordingly.