Through indirect measurements, astronomers know that it's out there, but they just can't «see» it. (science.howstuffworks.com)
They also correlated the amount of energy found in the sample to its chemical oxygen demand (COD), a commonly used indirect measurement of dissolved organic compounds. (scientificamerican.com)
Audi takes a rather unique approach to this mandate using indirect measurement rather than individual tire sensors and the upside is significant. (bestride.com)