As mentioned above, starting fresh with a brand new Apple ID is recommended. I'd go one step further and create an entire brand new email address utilizing a unique password unknown or not easily guessed by anyone. If you create a new Apple ID using an email address the person you're worried about knows, and they either have access to or can easily gain access to that email address, there may still be some vulnerability there.
Start fresh top to bottom. Brand new email address, wipe the device, create a brand new Apple ID using that brand new email address.
As far as being able to tell if the phone is tracked, there really isn't a definitive way but you can access "Find My iPhone" and "Find My Friends" to see if your location is being broadcasted to any friends and family members.