The location for the NFC reader on a standard iPhone and Android is as per the below diagram.
Personally, I tend to avoid using spectators with an Android phone as there are many different models of phones by different manufacturers, meaning the exact location of the NFC Reader can be unreliable.
iPhones are easy to spot and provide a more reliable detection area for the NFC.
Since NFC tags only work at close range, they tend to perform best on phones that aren’t in cases. So, when choosing a spectator’s phone during your routine, it’s a good idea to go for one without a case if possible as you’ll get more reliable results that way.
iPhones from 2018 onward support background NFC tag reading, so make sure the borrowed iPhone is at least an iPhone XS or newer. As a quick tip, the latest models have flat edges and no home button, which can help you identify them at a glance.
Keep in mind that iOS introduces a short delay between reads, so if you’re practicing, wait at least 10 seconds after scanning a tag before trying again to avoid the cooldown limitation.
Finally, always make sure the borrowed phone is set to silent. This avoids the NFC chime or any other distracting notifications during your effect. On iPhones, just flick the switch on the left side of the device toward the back to mute it.
It should also be noted that on iOS using an NFC tag will leave an entry in the notification history of the device. However, I have never been questioned or had a spectator bring this up.