The only way to tell if someone has read an e-mail in Outlook is to request a read receipt. When the recipient displays the message on his screen, his mail client will notify the sender that the message has been read.
Request a Receipt on Every Message
You can set Outlook to request a receipt for every e-mail sent, by going to "Tools," then Options," and clicking "Email Options" on the "Preferences" tab. From there, select "Tracking Options" and put a check in the box next to "Request a Read Receipt."
Request a Receipt for Individual Messages
Request a receipt for a single message by starting a new message and clicking the arrow next to "Options" on the Outlook ribbon. In pre-2007 versions of Outlook, the user clicks "View" then "Options," and selects "Voting and Tracking Options." The user then puts a check in the box next to "Request a Read Receipt."
The "Request Receipt" function only works if the recipient is using a mail client that sends receipts. Also, if the software prompts the user to send a receipt, he may opt not to send one. Finally, the read receipt only verifies that the recipient has displayed the email, not that he has actually read the message.