You can't. If you could someone could pick up your iPad and steal your passwords.
What email provider are you using?
If it's MobileMe, then go to me.com on a browser (preferably on a computer). When you try to sign in, and fail, you'll get a forgot password link. Follow that and the instructions that follow.
You can try the same thing for any other email provider, if they have web based access.
If that does not work you'll have to contact their support and ask for a password reset. You'll probably have to answer a few questions to prove you are who you are.
Normally you get password resets through email, but in this case it might be a tad difficult.
And when you are done you may want to get a password app. There are several in the App Store, and have saved me from many a headache.