Yahoo e-mail is a Web-based e-mail service, which means that you log onto your Yahoo mail online and can do so from any computer with Internet access. With Web-based e-mails, most of the information is stored online, and because the mail isn't provided by an Internet Service Provider, it is hard to get information on account logins. If you are concerned that someone has read your Yahoo e-mail, there are a few things you can try to see if this is true.
Log into your e-mail account. If you have messages in bold, these are new or unread messages. If there are messages that are not in bold, signifying that they have been read, and you have not opened or read them, you can know that someone else read your Yahoo e-mail.
Check through your sent folders. If someone read your messages, they might have also sent out messages. If you see messages that you did not send in your sent folder, there is a good chance that someone read your Yahoo mail.
Contact Yahoo through the help desk and explain that you believe your account has been hijacked or phished. In some situations, the company can give you access to login records, so you can look at the records compared with when you were online. If your account was logged into by someone else, you can be pretty sure someone read your e-mail.
Check the history on your computer or on the computer where you feel your e-mail was read. See if Yahoo Mail appears in the history. Compare the times on the history with the times when you know you checked your e-mail to see if someone was in Yahoo when you weren't at the computer.
Yahoo mail has a feature where you can read an e-mail and then mark the email as "unread." Therefore, unless you go through legal channels to request login records, you might not be able to tell definitively whether someone read your e-mail.