Q: How do I know if the primary person on my iPhone account is hacking into my phone?
A: Can they “hack” your phone? Not unless they have a spare $15,000-$1M US hanging around, or they know your PIN. However, if you have jailbroken the iPhone, all bets are off. By definition, you have removed all security from your device.
But… you state “the primary person on my iPhone account”. Normally, the “primary person” is the owner of the account, and possibly the phone itself. If this is the case, it is not “your” phone or account, it is theirs. This may give them access to the Find My iPhone feature in iCloud to track the location of the phone. In the case of a business phone, they may have the legal right to full access.