I think someone has gotten my yahoo email password and is getting into my email, how can I prove it?

Is there anyway I can tell if they are not using my computer and isn't this highly illegal? If I find out who it is what can be done?

I think someone has gotten my yahoo email password and is getting into my email, how can I prove it?
Open another new e-mail address add your old emailaddress on yahoo messenger if that person signs in your email adress then you will know as he signs in then chat with him..with atrick ask who the person is..if incase is your loved one..write alove letter asuming is some otherlover he will prompt you who is that....or if you think you email is at risk change the password.
Reply:change your password duh
Reply:Just change your password. That will end it if it is occuring.