If you suspect tampering there is no way to examine and be certain there is nothing concealed. The safe option is to nuke it back to factory condition Restore your iPhone, iPad, or iPod to factory settings - Apple Support and setup the phone from scratch. Before you do this, please see Turn off Find My iPhone Activation Lock - Apple Support and make sure YOU have the password and username to remove the phone from its Apple ID, or you may find it can never be reactivated. You could also consider making an entirely new Apple ID when you reinstall the phone so you are sure you are not sharing everything with your ex - or as minimum change the password and security questions on the Apple ID.
Post back with more questions if you need help, particularly on the Apple ID thing. Please DON'T start erasing things until you have gained access to the Apple ID account, because one of the easiest routes to changing the password needs you to have the iPhone in full working condition.