Meaning of “someone who can smell an expense account at forty paces”

The phrase "smell smth. at long way away" means to be able to detect something. It implies that the person is very good at telling that someone has some property or just that something exists.

In your particular case it means the people who can tell someone has an expense account they can use to buy dinners or similar on expenses for the company they work for. It could also be talking about a job candidate who knows that a given position will afford them an expense account.

Forty paces is just one of the possible choices You can often hear "I can smell a liar a mile away" for example.