Suppose you look at a spectrum of visible light by looking through a prism or diffraction grating. how can you decide whether it is an emission line spectrum or an absorption line spectrum?

"An emission spectrum consists of bright lines on a dark background, while an absorption line spectrum consists of dark lines on a rainbow background."

Consider how each spectrum is formed.

Emission Spectrums:

Electrons lose energy when they transfer within an atom from an energy level with high energy to one with lower energy. That energy will be released as a beam of light on one single frequency. The frequency of the light depends on the size of the energy change. There can be an infinite number of transitions. However, only a few of them release light in the visible spectrum. Other parts of the spectrum will end up being dark.

Absorption Spectrums:

A beam of white light contains light of all frequencies with a rainbow spectrum. Light of each frequency carries a specific amount of energy. Electrons in an atom will absorb a frequency if it provides the same amount of energy as the energy differences between two energy levels in that atom. The electron will then move to a level with higher energy. Only a few of those energy differences absorb light in the visible spectrum. Those frequencies will appear as dark lines, while the rest of the spectrum will continue to show a rainbow.