| Things like keyloggers log every stroke that is made on the keyboard and can be set up to add all occurences into a log file specified by the user. This needs to be installed however, before the event takes place. Incidentally, most a/virus or spyware solutions pick up keyloggers as offending items.
There are programmes that can extract hashed passwords from say, the SAM database files in Windows and then other apps that will crack the hashed files. But again, these are mainly for passwords that are stored in your system. If somebody uses a log on for a website, the password details are stored on the server that hosts the website, not in your machine. I don't need to describe the complexity of how this information would need to be obtained.
I think what you are trying to do would require a certain amount of technical expertise as you are getting into the realms of computer phorensics etc. You could try checking the clipboard as you would be surprised at how many people use the same password for multiple purposes and also copy and paste it. However, if you have started a new session since the event, this would not really be successful either. If the person has checked the box asking if you want Windows to remember the password then it will be on your machine. Search Google as there are quite a variety of tools that can be used to extract this info, depending on your OS. | 
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