How can you prove someone has opened my emails?

Normally you can prove it by color difference, when you open a e-mail from the inbox it goes from white to a dark grayish color depending on the e-mail provider you use. And if you dont remember clicking on that e-mail and its gray not white which gray means read, then you know someone is reading them. And if you really want to take it farther and catch them. I suggest setting up a at home keylogger, which will tell you what everyone does on the computer so you will read the information and it will say they was logging into your e-mail, checking certain stuff etc.. etc..