I don't think there are any "legal" means of doing so. Best you could try is to google the screen name and see if it turns up elsewhere. Other than that would probably involve some sort of third party hacking tool(s).
you can't Improved: You can find out who someone is if you enter the screen name they are using into Google. If they've used that screen name before on different websites, it will pull up. If they've...
1. Ask.
Ask them or someone who knows them.
2. Use Linux, or a program ported from Linux to windows:.
I used a program called gaim on all my Linux boxes that has a feature where you can search for...