There is one way out. You can make a fake email address and then email her on the account. If the mail comes back to you with a failed delivery notification, that means the account has been deleted. Otherwise, she will receive an email.
How Can I Tell If My Yahoo Account Has Been Deleted ?
As far as I know, I don't think Yahoo! Accounts can be deleted!! Once an account is opened in Yahoo!...it cannot be closed. What you can do is, tell your wife to sign in to her account in front of you. I am sure she will be signed in properly if she gave the accurate password for it and not a fake one. Once she is signed in, you go to her account and change her password. Let the new password be something very difficult..like for example, choosing randomly from a book containing name of cases, etc. In one word, choose something 'difficult' which she would never guess and it is also not necessary for you to remember. By the way, don't make her feel like a criminal. It may hurt her feelings and I am sure you won't like it as it shows that you love her a lot and want her to be yours only. Best of luck! :)
There is a special page to delete your a/c. Ask a question in the answers category.If you ask, one of the Top Contributors will give you a link. Its not fully deleted for several months,depending on how much you had to do with them. I know thats true as I have just done it!
You can delete a yahoo account. Not signing in for more that 30 days makes the account inactive. Contact the administrator about it. Or you can do as Aicha suggested and send an email from a email address she doesn't know that will let you know. Especially if she answers. Good luck.