IsItPrivate.cs

namespace ConsoleApplication2
{
    using System;
    using System.Linq;
    using System.Reflection;

    class Test
    {
        string @private;

        internal string @internal;

        public string @public;
    }

    class Program
    {
        static void Main(string[] args)
        {
            var info = typeof (Test).GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
                .FirstOrDefault(f => f.Name == "private");

            Console.WriteLine(info.IsPrivate ? "Its private" : "Its not");
        }
    }
}