C#

static void Main(string[] args)
{
    var portName = @"\\.\COM3";
    var handle = CreateFile(portName, 0, 0, IntPtr.Zero, 3, 0x80, IntPtr.Zero);
    if (handle == (IntPtr)(-1))
    {
        Console.WriteLine("Could not open " + portName + ": " 
            + new Win32Exception().Message);
        return;
    }
    var type = GetFileType(handle);
    Console.WriteLine("File " + portName + " reports its type as: " + type);
}

[DllImport("kernel32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall,
    SetLastError = true)]
public static extern IntPtr CreateFile(
    string lpFileName,
    uint dwDesiredAccess,
    uint dwShareMode,
    IntPtr SecurityAttributes,
    uint dwCreationDisposition,
    uint dwFlagsAndAttributes,
    IntPtr hTemplateFile);

[DllImport("kernel32.dll")]
static extern FileType GetFileType(IntPtr hFile);
enum FileType : uint
{
    UNKNOWN = 0x0000,
    DISK = 0x0001,
    CHAR = 0x0002,
    PIPE = 0x0003,
    REMOTE = 0x8000,
}