How do I tell if keyboard input is coming from a barcode scanner?

Take a look at Microsoft's MultiPoint SDK

(edit: this answer is no longer applicable now that the question has been clarified. i'm leaving it here for others to discover though)

Almost all barcode readers can be configured with a prefix and suffix to whatever it reads. Try and configure yours with, for instance, a prefix of "*" and a suffix of <TAB>, and then in your C# code, force the focus to an invisible textbox whenever * comes from the input stream, and in the lostfocus event of this textbox put all code to process the entry. Be aware though that the character you chose to be the prefix is never entered in the keyboard. Also, set the tabstop property of the textbox to false, just to keep the user from reaching the object when navigating the screen.

You'll get input from both. Not simultaneously, of course. It will all be placed into a queue, but Windows will process key events from both keyboards.

Don't be helpless, though. As David Heffernan suggests, you can easily figure this out yourself by plugging in both keyboards to your computer, opening up Notepad, and typing random characters to see which one generates input.

You reply that you want to "check that with C# code", but I have no idea what that means. How about creating a console app that reads input from the keyboard and displays it on the screen?

using System;

class AdvancedKeyboardTester
{
   static void Main(string[] args)
   {
      for (; ;)
      {
         Console.ReadKey();
      }
   }
}

Press Ctrl+C when you tire of the fun and want to quit the program.

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Edit: It sounds like you're looking for the RegisterRawInputDevices function, which allows you to enable raw input for all of your keyboards, and then enumerate through the results to determine which device sent the message.

Fortunately, it looks like someone has already written a C# wrapper library for this, available for download on Code Project: Using Raw Input from C# to handle multiple keyboards

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Edit 2: (it seems the information just keeps tricking in from the comments)

If you're using a barcode scanner, this gets a lot easier. Because they're explicitly designed for this purpose, they're almost all programmable. Meaning that you can tell them to prefix (and/or suffix) their input with some sentinel characters that indicate the input is coming from the barcode scanner, rather than a standard keyboard. (Check your barcode scanner's user manual for more information.) Then, all you have to do is filter out the keyboard input based on the presence or absence of those sentinel characters. You can also check for how quickly the characters between the prefix and suffix were entered.