How to figure out whether a business action(method) should reside in domain object(class) or domain service(class)?

According to "Domain Driven Design",domain service encapsulates business logic that doesn't naturally fit within a domain object.The definition of domain service is clear,but how can i distinguish whether a business action belong to a domain object or domain service.For example i have a Account class,and a business action call transfer,both domain service and domain object can finish the transfer task.which one should i choose?

public class Account
{
  public void Transfer(Account bar)
  {
     //do something
  }
}
public class AccountService
{
  public void Transfer(Account foo,Account bar)
  {
     //do something
  }
}