There used to be an official Apple page where you could check the phone's Activation Lock Status, but it was summarily removed cause the miscreants were somehow abusing the feature. So anything mentioning that page is outdated and of historical nature. If you're in a face-to-face with the seller, just watch them carry out each of the steps listed on top here (can skip #2 if they don't want): What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support
If they cannot complete them, then there's something dodgy on the phone. Check also here: Find My iPhone Activation Lock - Apple Support