Are you using lotus notes? If so, your IT dept can determine when an email was first opened, though they probably wouldn't tell you!!! Worth ringing and asking though! Once its marked as unread, you won't be able to tell she has read it though
The way the law is, your email address is a work facility, it belongs to the company in the same way that your computer, desk etc does and your manager is entitled to read your emails
WHY she is reading them is a different matter, if it is to ensure that work isn't being missed on your free days, fair enuf but if this is the case, you'd think she would have raised it with you at some point. If she thinks you are abusing the system, then yes, thats fair enuf
If however, she is just being plain nosey or is trying to cause problems for you, then I would discuss it with her manager, union rep, HR or someone