If someone has my Itunes password and phone number/email address, are they able to set-up their iphone/ipad to see my imessages without my consent? If so, is there a way for me to trace this?
Yes, in a way, it is possible to "trace" it.
The good news for you, is that (I'm speaking in regards to Mountain Lion and iOS 6, not necessarily for older versions) every time a device authorizes and uses your Apple ID and phone number it will alert you on all your other devices. For example, if you are signed into your Apple ID for Messages on your Mac, and you purchase a new iPad Mini, and sign into it, it will display an alert on your Mac that says (something like) "[Your] iPad Mini is now using [your Apple ID] for iMessage". There is no way for you to deny that request (it is just informational) but that would alert you to possible abuse of your account if someone else has maliciously set up your Apple ID on your account. With this information you can easily change your Apple ID password and render the abusive user's information useless.
I'm not aware of a way to view all of your devices that are using your Apple ID for Messages and Facetime, but if you are careful to be aware of those prompts that come up, and change your password if something suspicious comes up, you will be set.