He would have needed physical access to your phone to install any sort of monitoring software. If you think that might be the case, you can go into Settings>Applications>Manage Applications or Running Services and look for apps that you don't recognize. Keep in mind there are some applications that are preinstalled that are important for your device to operate properly, so don't go deleting things willy-nilly. If you do suspect monitoring software has been installed, you should consider performing a factory reset. Make sure you back up things you don't want to lose before you do it. This will revert your phone to the state it was in when you got it, and you will have to set it up again and reinstall all of your apps.
However, probably the most common way people "stalk" others is by getting access to their Google account. This will allow someone to see where you have been, read your messages, etc. To be safe, I would change my Google password and set-up 2 factor authentication to make it much harder form someone to log into you account without authorization. Make sure to change your security questions as well - if someone knows you well, they can probably answer most of the standard ones (where were you born, where did you go to elementary school, etc.).