Answer:
By measuring the speed of light in both in space and that unknown substance
Step-by-step explanation:
Refractive Index (Index of Refraction) is a value calculated from the ratio of the speed of light in a vacuum to that in a second medium of greater density.
By measuring the speed of light in that medium and dividing it by speed of light 3*10^8 we can measure the refractive index of any medium