Make a prediction for how to identify an unknown element and the chemical composition of a star

The answer is: measuring specific emission spectrum of elements using the spectroscope.

The chemical composition of stars can be determined, because each element emits or absorbs light only at specific wavelengths.

Emission spectrum of a chemical element is the spectrum of frequencies emitted due to an atom making a transition from a high energy state to a lower energy state.

Each element's emission spectrum is unique.