"'And yet two thousand a year is a very moderate income,' said Marianne. 'A family cannot well be maintained on a smaller....A proper establishment of servants, a carriage, perhaps two, and hunters, cannot be supported on less.'

Elinor smiled again, to hear her sister describing so accurately their future expenses at Combe Magna.
'Hunters!' repeated Edward - 'But why must you have hunters? Every body does not hunt.'
Marianne coloured as she replied, 'But most people do'"

So when Mrs Jennings and Sir John talk about Willoughby and Marianne in a way that raises Edward's suspicions, he guesses that Willoughby hunts (:D)