AssertNullChecked.cs

void Main()
{
    AssertNullChecked(() => new List<int>(null));
}

public static void AssertNullChecked(Expression<Action> expr)
{
    (IReadOnlyCollection<Expression>, IReadOnlyCollection<ParameterInfo>) GetArgumentsAndParameters()
    {
        if (expr.Body is MethodCallExpression callExp)
        {
            return (callExp.Arguments, callExp.Method.GetParameters());
        }

        if (expr.Body is NewExpression newExp)
        {
            return (newExp.Arguments, newExp.Constructor.GetParameters());
        }

        throw new NotSupportedException($"Unsupported expression type: {expr.Body.GetType()}");
    }

    var (args, parameters) = GetArgumentsAndParameters();
    var (_, nullParameter) =
        args.Zip(parameters, (arg, param) => (arg, param))
        .Single(x => x.Item1 is ConstantExpression constant && constant.Value == null);

    var ex = Assert.Throws<ArgumentNullException>(expr.Compile());
    Assert.Equal(ex.ParamName, nullParameter.Name);
}