Program.cs

using System;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            // Want to detect which build is used
            Console.WriteLine(BuildConfiguration.IsDebug ? "Debug" : "Release");
        }

        public static class BuildConfiguration
        {
            /// <summary>
            /// Gets a value indicating whether the assembly was built in debug mode.
            /// </summary>
            public static bool IsDebug
            {
                get
                {
                    bool isDebug = true;
#if !DEBUG
                isDebug = false;
#endif
                    return isDebug;
                }
            }

            /// <summary>
            /// Gets a value indicating whether the assembly was built in release mode.
            /// </summary>
            public static bool IsRelease
            {
                get
                {
                    return !IsDebug;
                }
            }
        }
    }
}