Answer:
Color
Step-by-step explanation:
The most common way of determine the temperature of a star is by studying its spectrum. The light coming from the star is measured in two filters: red and blue. The ratio of the two readings give us the temperature of the stars.
Cooler stars will appear red in color and hotter stars will appear blue.
For a layman if the star is blue it is a hot star and if its red it is a cold star.