How to test a delegate using Moq and NUnit

I have a factory class returning a delegate like below (GetDelegate method)

  public interface IFactory
{
    Func<int, string> GetDelegate(bool isValid);
}

public class AFactory : IFactory
{
    private readonly IService1 _service1;
    private readonly IService2 _service2;

    public AFactory(IService1 service1, IService2 service2)
    {
        _service1 = service1;
        _service2= service2;
    }

    public Func<int, string> GetDelegate(bool isValid)
    {
        if (isValid)
            return _service1.A;
        return _service2.B;
    }
}

public interface IService1
{
    string A(int id);
}

public interface IService2
{
    string B(int id);
}

I have been trying to write unit test for GetDelegate but dont know how to assert that a specific Func was returned depending on isValid

My attempt of a unit test is like below (which i am not happy with)

[Test]
    public void ShouldReturnCorrectMethod()
    {
        private var _sut = new AFactory(new Mock<IService1>(), new Mock<IService2>());

        var delegateObj = _sut.GetDelegate(true);
        Assert.AreEqual(typeof(string), delegateObj.Method.ReturnType);
        Assert.AreEqual(1, delegateObj.Method.GetParameters().Count());
    }

Any help is much appreciated

thanks