Question

Are the most reliable indicator of what the public is thinking, and a whole industry and science have grown up around measuring opinion on everything from presidents to toothpaste.

Answer 1

Surveys of public opinion or polls are the most reliable indicator of what the public is thinking, and a whole industry and science have grown up around measuring opinion on everything from presidents to toothpaste.

Answer 2

The answer to this is "the polls", on in other words "surveys of public opinion". 

The polls can for example look as follows: 

51% of people prefer blue to red, and 49% prefer red to blue - this can for example inform toothpaste companies how to design their packaging. 

Polls also help predict the result of political elections.