This is how I would test that a decorator delegates to the wrapped object in FakeItEasy.

gistfile1.cs

public interface IRobot
{
    string Name { get; set; }
}

public class RoboCop
    : IRobot
{
    private readonly IRobot robot;

    public RoboCop(IRobot robot)
    {
        this.robot = robot;
    }

    public string Name
    {
        get { return string.Concat("Officer ", this.robot.Name); }
        set { this.robot.Name = value; }
    }
}

public class RoboCopTests
{
    [Test]
    public void Should_set_name_property_to_wrapped_object()
    {
        // Arrange
        var wrapped = A.Fake<IRobot>();
        var cop = new RoboCop(wrapped);

        // Act
        cop.Name = "John";

        // Assert
        Assert.That(wrapped.Name, Is.EqualTo("John"));
    }
}