If you suspect someone else is reading your emails you normally change your password immediately and figure out how they were able to access your account.
If you’re curious then the following information could interest you 😉
There’s a free online service called OneStatFree that can be used as a tripwire to detect access to your emails. It will tell the time and day your email was opened (by someone other than you), the country it was access from, the IP address and possibly more information (such as city) depending on the actual network used.
The way it works is you create a special email and send it to yourself. You never open this email yourself and if someone else does it will instantly send some information to the OneStatFree service, which you then check at a later date.
Full instructions are provided here, it should be fairly easy for most people to follow.
Just keep in mind that if someone is indeed reading your emails this trick won’t stop them. So think carefully if you want to continue compromising your email while you investigate the culprit, or take immediate action and change your password.