Yahoo have a new facility to confirm the yahoo site
click on "prevent password theft", if it is not there refresh the page (try F5 or ctrl+F5)
selet "create a text seal" and enter a text or select "Upload an image" and upload a image for the seal
you can preview the seal and save if it is ok
click make sure for more
This is cookie based you will have to enable cookie for this (Yahoo mail will not work with out cookie)
This will be appear only in the machine you created.
Read it from yahoo
How to make sure you're really on Yahoo!.
When your sign-in seal is unavailable and you wish to sign in to Yahoo!, be sure to look carefully at the Address bar of your browser. A genuine Yahoo! sign-in page will always display https:// at the beginning and then .yahoo.com just before the next forward slash.
Also look for the lock icon. Together, these are good indicators that you're on a true Yahoo! site.
Why is this important?
Whenever you wish to sign in to Yahoo!, be careful that you're not on a "phishing" site--one that looks like a Yahoo! page but is designed to fool you into revealing your password or other confidential information.
Beware of links from emails or IMs.
When you see a sign-in page after clicking on a link from an email or an IM, even from a friend, check the Address bar. This is a common way to trick you into signing in to a phishing site.
If the Address bar looks wrong...
You might have been directed to a phishing site. If you're at all uncertain, don't sign in on that page. Here's the safest way to sign in to Yahoo!:
Close and restart your browser.
in the Address bar.
On that page, click the Sign In link and sign in to Yahoo!.
My Address bar looks OK. Why is my sign-in seal missing?
It's possible that:
someone else using this computer may have changed it.
you haven't created one on this computer. Remember, you should create a seal for each computer you use regularly.
your cookies or files on your computer may have been deleted.
When you're sure you're on a genuine Yahoo! sign-in page, you may go ahead and create a new seal.