If you suspect your privacy has been compromised, it's best to change your password immediately. However, it'll be nearly impossile to prove that someone else has been reading your e-mail.
If you had one of their paid service (Yahoo! Mail Plus, SBC Yahoo!, Verizon Yahoo!, etc), you could have them monitor access to your account, and possibly even go back and check the history of the account for date and times of access. However, with the free service, account access isn't usually monitored, unless a court order is issued. (You could always call Yahoo! Inc and ask, but I doubt you'd have any luck.)
The best you could do is check your browser's history and see when Yahoo! Mail was last accessed. While it won't state what account was accessed, it would give you a date and time. If you weren't the one who logged in, and she doesn't have a Yahoo! account, then you'll know. However, that's far from proof, and is only an option if you suspect she is accessing your account from your computer.
Hope this helps,