By Salesforce Id
This code block retrieves a Contact record from Salesforce
ContactService contactService = new ContactService(salesforceSession);
//This is the Id for a contact directly from the browsers address bar
Id contactId = new Id("00390000001TjMHAA0");
Contact contact = contactService.GetByEntityId(contactId, new SObjectField[] {Contact.Fields.Id, Contact.Fields.FirstName, Contact.Fields.LastName});
Assert.IsNotNull(contact);
Assert.AreEqual(contactId, contact.Id);