Q:

Android: How to tell if two phones are within approximately a foot of each other

Looking for some ideas on how to tell if two Android devices are within a foot of each other. 
GPS doesn't seem to be accurate enough. 
NFC needs to be way too close. 
Maybe observing the bluetooth strength connection?  How do bluetooth beacons do it? 
Any ideas to get my brain thinking would be appreciated! 

A:

Bluetooth beacons works on proximity, for this to work you should need to have one of the phones broadcasting as a beacon while the other listening to beacon activity. 
Although reading just now, Beacon broadcasting is for Android5.0+
It might be good to check bluetooth capabilities.