Unless you have shared any of your mail folders, you cannot tell from Outlook if anyone is reading your e-mail unless you have permissions to access the e-mail server and supporting programs.
It appears you are asking from a corporate setting. If there is an e-mail policy in place, it will define how e-mail is handled at your work place and who may have access to it. It is a trivial matter to give people access to your e-mail, if, by policy, they are allowed that access.Most managers do not have the time to give more than a cursory glance to such mail access, unless than there is a reason for the manager to pay attention.
As already mentioned, a corporate account should be used for corporate purposes, not for personal e-mails to mom or your girlfriend, arranging next weekend's softball game and bar-b-q (unless it is the company team) and so on.
You should have nothing to fear if a manager or higher it looking at your e-mail, it may even help you advance.