know which way the wind blows

To be able to anticipate how a certain course or situation is likely to develop. These politicians know which way the wind blows regarding public opinion on the issue, and you can bet they'll vote accordingly.

How matters stand. Wind direction has been a metaphor for the course of events since the fourteenth century. It appeared in John Heywood’s Proverbs of 1546 and remains current. “My questions must have showed him whence the wind blew,” wrote Sir Arthur Conan Doyle (Sir Nigel, 1906). See also straw in the wind.