How can I see all keypresses on the keyboard?

What you need is a Keylogger. For example, you can use Actual Keylogger, a free keylogger which runs hidden in the background and automatically records all keystrokes.

Alternatively, you can use PassMark Keyboard Test. It's a shareware with a fully functional and free 30 day evaluation. It allows you to quickly check that all the keys on your computer keyboard are functioning correctly, check the keyboard LED's and look at the internal scan codes being generated by the keyboard.

You could use AutoHotkey and KeyboardHook to see what keys are being pressed. You only need to create a script with this line

#InstallKeybdHook

Save the file with and "ahk" extension and run it. Then in the Systray do a right click in the proper icon and select open. In the menu select View->History.....press F5 to update. This two sites could be of some help:

http://www.autohotkey.com/docs/commands/_InstallKeybdHook.htm

http://www.autohotkey.com/docs/commands/KeyHistory.htm

A really quick Google search turned up this: http://keyboard-tester.software.informer.com

I can't add a comment to mjsr but just wanted to follow up and say that his answer was incredibly useful for me. In my case I had to also add the line:

#KeyHistory 200

As I was trying to track the differences that were occurring between two devices when a QR Code was being scanned. The default #KeyHistory was only around 50 characters, whereas my QR contained around 150.